Document:

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                                                                 Exhibit 10.15.1

[LOGO OF VIA]

May 26, 1999

Antonio Tavares
Rua Bandeira Paulista, 716--1st floor
04532-002--S. Paulo
BRASIL

Dear Antonio:

Pursuant to our conversation, I am pleased to offer you the position of Vice
President, Latin America Region with VIA NET.WORKS, Inc. ("VIA") reporting to
me, beginning on May 11, 1999. Your annual salary will be R$215,000 or
R$17,916.67 for each of the 12 months. You will also be eligible for an annual
bonus of up to 60% of your base annualized salary during the calendar year 1999,
and up to 50% of your base annualized salary thereafter. VIA will administer
your payroll through one of its affiliate companies in Brazil.

You have been approved for a stock option grant of 75,000 shares of VIA
NET.WORKS, Inc. Common Stock at the fair market price as established by the
Board of Directors. Options vest over four years, the details of which will be
included in the formal grant. You have also been granted the right to purchase
up to 35,000 shares of Common Stock at the same fair market value within 60 days
of Board approval. Plan and grant documents will be forwarded to you shortly.
Your vacation will be 30 days per year and you will be eligible for personal and
sick leave as set forth in VIA's corporate policies.

In your position with VIA, you may have direct or indirect access to a
substantial amount of confidential material/data, patentable or unpatentable
inventions, trade secrets, technical know-how etc., and you will be required not
to divulge these during or subsequent to your employment with VIA. Accordingly,
you will be requested as a condition of your employment to sign an Employee
Non-Disclosure Agreement.

If the foregoing is acceptable to you, please sign below. We look forward to
having you join us.

Sincerely,

/s/ David M. D'Ottavio

David M. D'Ottavio
President and CEO                     Read, Approved and Accepted

                                      By: /s/ Antonio Alberto Valente Tavares
                                         -------------------------------------
                                         Antonio Alberto Valente Tavares<PAGE>

                                                                Exhibit 10.15.2

[VIA LOGO]
VIA
NET.WORKS

October 21, 1999

                                                VIA FEDERAL EXPRESS

Antonio Tavares
Rua Bandeira Paulista, 716 - 1st floor
04532-002, Sao Paulo
Brasil

Dear Antonio:

In addition to the matters covered by the Termination Agreement dated October
21, 1999, and in order to complete your transition from CEO of Dialdata Internet
Systems S.A. to Vice President, Latin America Region of VIA NET.WORKS, Inc., we
need to make sure that certain provisions of your Retention Agreement with
Dialdata Internet Systems S.A. dated December 29, 1998 remain in effect.
Specifically, we would like to confirm your agreement that notwithstanding the
termination of the Retention Agreement with Dialdata, for the purposes of your
employment with VIA, Section 09 relating to your agreement not to compete with
Dialdata for a period of eighteen (18) months after you cease to be an officer
of Dialdata, shall apply in full effect upon the termination of your employment
with VIA as follows:

     SECTION 09.    From the date hereof until eighteen (18) months following
     ----------
     the date on which you cease to be an officer of VIA NET.WORKS, Inc. (the
     "Restriction Period"), neither you, nor any of your relatives up to second
     degree or any entity with respect to which you, directly or indirectly,
     individually or collectively, owns more than (thirty percent) 30% of the
     total voting securities or other ownership interests (each a "Subsidiary"),
     shall:

         (a) directly, or indirectly through any other individual or Subsidiary:
     (i) own, (ii) manage (as an employee, consultant or otherwise), (iii)
     operate, (iv) render services to, (v) become interested in or associated
     with, (vi) join in, (vii) control (by virtue of ownership of voting
     securities, contract or otherwise), (viii) participate in, or (ix)
     otherwise carry on any Competing Business as hereinafter defined. For
     purposes of this Agreement, a "Competing Business" shall include any
     entity, person, individual or business other than Dialdata Internet
     Systems, Inc. ("Dialdata") engaged in the activity of providing internet
     connectivity and value
<PAGE>

     added services, including, but not limited to, internet access, domain
     registration, web hosting, web site management, intranet, extranet,
     electronic commerce and virtual private network services in Brazil;

         (b) directly, or indirectly through any other individual or Subsidiary,
     enter into any employment agreement or contractual relationship of any
     nature with or related to any Competing Business, except with VIA
     NET.WORKS, Inc. or any affiliate of VIA NET.WORKS, Inc.;

         (c) directly, or indirectly through any other individual or Subsidiary,
     solicit, entice, persuade or induce any Employee, as hereinafter defined,
     to: (i) terminate or refrain from renewing or extending his or her
     employment with Dialdata; or (ii) enter into any contractual relationship
     with any Competing Business. For purposes of this Agreement, "Employee"
     shall mean and include all persons employed by Dialdata on the date hereof
     or at the time of taking of any of the actions prohibited by this Section
     1(c);

         (d) directly, or indirectly through any other individual or Subsidiary,
     solicit, entice, or induce any Customer or Supplier, as hereinafter
     defined, to terminate, reduce or refrain from renewing or extending its
     contractual or other relationship with Dialdata, to become a customer of or
     to enter into any contractual or other relationship with any Competing
     Business. For purposes of this Agreement, "Customer" shall mean and include
     all individuals or entities that acquire services from Dialdata during the
     Restriction Period, and "Supplier" shall mean and include all entities from
     whom Dialdata purchased or leased services, goods or other supplies during
     the Restriction Period.

Please indicate your agreement by executing a copy of this letter and returning
it to me.

Thank you,

Sincerely yours,

/s/ Matt Nydell
Matt Nydell

Vice President, General Counsel & Secretary
VIA NET.WORKS, Inc.

                                       Agreed:

                                       /s/ ANTONIO ALBERTO VALENTE TAVARES
                                       ________________________________________
                                            ANTONIO ALBERTO VALENTE TAVARES

                                       2<PAGE>

                                                                 Exhibit 10.15.3

                                Exhibit 10.15.3
                                ---------------

--------------------------------------------------------------------------------

                              RETENTION AGREEMENT

                                BY AND BETWEEN

                        DIALDATA S.A. INTERNET SYSTEMS

                                      AND

                       ANTONIO ALBERTO VALENTE TAVARES,

                         DATED AS OF DECEMBER 29, 1998

--------------------------------------------------------------------------------
<PAGE>

                              RETENTION AGREEMENT

By this private instrument, on one side, Dialdata S.A Internet Systems, a
sociedade anonima, with its registered office at Rua Bandeira Paulista, 716, 1st
floor, in the City of Sao Paulo, State of Sao Paulo, enrolled with the General
Taxpayers' Roll of the Ministry of Finance (CGC/MF) under No.
69,286,540/0001-80, herein represented by its undersigned Officers, hereinafter
referred to as RETAINER and, on the other side, Mr. Antonio Alberto Valente
Tavares, Portuguese citizen, married, businessman, resident at Alameda Diamante,
454, in the City of Santana do Parnaiba, State of Sao Paulo, bearer of Brazilian
Identity Card RNE No. W583,350-Q and enrolled with the Individual Taxpayers'
Roll of the Ministry of Finance (CPF/MF) under No. 429,270,997-15, hereinafter
simply referred to as OFFICER;

WHEREAS, pursuant to the terms and provisions of the Purchase Agreement dated as
of December 29, 1998, V-I-A Internet Brasil Holdings Ltda. ("V-I-A Brazil") is
purchasing from the OFFICER and other shareholders of RETAINER a portion of the
common shares that they own in RETAINER's capital stock and also subscribing
common shares of RETAINER's capital stock;

WHEREAS, the OFFICER was appointed Chief Executive Officer of RETAINER, pursuant
to resolution of the General Shareholders' Meeting of RETAINER held on the date
hereof;

NOW, THEREFORE, in consideration of the agreements contained herein, the parties
hereto agree as follows:

SECTION 01.     The OFFICER is hired to render services to RETAINER in
----------
the capacity of Chief Executive Officer, pursuant to RETAINER's Bylaws. The
OFFICER shall thus perform, on an exclusive basis, the duties of Chief Executive
Officer and shall comply with the tasks compatible with such duties, abiding by
the principles and guidelines set forth in the Brazilian Corporation Law (Law
No. 6,404/76, as amended), in RETAINER's Bylaws and established by the General
Shareholders' Meetings of RETAINER held from time to time.

                                       2
                 [Retention Agreement Chief Executive Officer]
<PAGE>

     Sole Paragraph.    As set forth in the Bylaws of RETAINER, the OFFICER
shall be responsible for overall operations and strategic planning of RETAINER,
as well as marketing and sales operations of RETAINER

SECTION 02.     The OFFICER is retained by RETAINER for a two (2) year period as
----------
of the date hereof, in accordance with the General Shareholders' Meeting of even
date, renewable by mutual agreement of the parties.

     First Paragraph.   The OFFICER may resign at any time from his position of
Chief Executive Officer, provided that a ten (10) day prior written notice is
forwarded to RETAINER to that effect.

     Second Paragraph.  The shareholders may at any time replace the OFFICER or
declare his position vacant at any RETAINER's Shareholders' Meeting, duly
convened and in accordance with the requirements of the Brazilian Corporation
Law. The OFFICER shall be advised of his replacement or his dismissal by means
of written notice given to that effect, provided that for all purposes,
including those of Section 04 below, the replacement shall be considered
effective as of the date of the Shareholders' Meeting at which decision on such
replacement is made.

SECTION 03.     The OFFICER, in consideration of the services hereby agreed
----------
upon, shall be entitled to an annual compensation of one hundred seventy two
thousand five hundred reais (R$ 172,500.00), payable in twelve (12) installments
of fourteen thousand three hundred seventy-five reais (R$ 14,375.00) each.

     First Paragraph.   The OFFICER shall further be entitled to a thirty (30)
day vacation period every twelve (12) month period, without any increase or
reduction of the compensation set forth in this Section.

     Second Paragraph.  The OFFICER shall be eligible for an annual bonus of up
to sixty per cent (60%) of his annual compensation as set forth in this Section
03. The bonus award shall be granted at discretion of the Ordinary General
Shareholders' Meeting of RETAINER and depend upon the OFFICER's overall
performance of his duties hereunder during the term of this Agreement, as per
goals and

                                       3
                 [Retention Agreement Chief Executive Officer]
<PAGE>

objectives attained in compliance with those established by the General
Shareholders' Meeting.

SECTION 04.     The OFFICER shall be entitled to the compensation set forth in
----------
Section 03 above and to the amounts corresponding to the benefits provided for
in its second paragraph, in each case calculated pro rata temporis, if, for any
reason whatsoever pursuant to such Section 02, he ceases to be the Chief
Executive Officer of RETAINER. Such payments shall be made by RETAINER to the
OFFICER within 10 (ten) days as from the date on which his resignation notice is
received by RETAINER or that of the Shareholders' Meeting at which his
replacement is decided, as the case may be.

     Sole Paragraph.    The Officer shall be entitled to the annual bonus
provided for in the Second Paragraph of Section 03 above in case a favorable
resolution in this direction is taken by the Ordinary Shareholders' Meeting,
even if this Agreement is terminated during the period between the date of such
resolution and the termination hereof.

SECTION 05.     The OFFICER hereby undertakes not to render services of any
----------
type related to the object of this Agreement or the activities of RETAINER to
third parties, except if RETAINER agrees, in writing, with such rendering of
services.

SECTION 06.     The OFFICER shall be liable for any acts or decisions that
----------
violate the provisions of the applicable Brazilian legislation, RETAINER's
Bylaws, the resolutions taken at the Shareholders' Meetings of RETAINER and this
Agreement.

SECTION 07.     The OFFICER agrees that all the business, technical, financial,
----------
personnel and other information related to present or future businesses of
RETAINER, of related or holding companies thereof, or holding companies,
subsidiaries, affiliates or companies subject to the common control of any of
them ("Related Companies") are valuable trade secrets and confidential
information of RETAINER.

                                       4
                 [Retention Agreement Chief Executive Officer]
<PAGE>

     For the purpose of this agreement, the term "trade secrets and confidential
information" shall also include, without limitation, operating processes,
software and computer files related to RETAINER, or Related Companies, and to
the business and prospects and strategies thereof, and information such as sales
and pricing data and procedures, client lists, business plans and marketing,
research and development plans, present and future, as well as the financial
statements thereof, and confidential information delivered to RETAINER or
Related Companies by the respective clients, suppliers, or agents thereof.

     The following information is also considered as "trade secrets and
confidential information": (This list includes but does not exclude other
examples).

(a)  Any information related to the particular clients, members/participants,
     businessmen or groups of RETAINER or Related Companies.

(b)  Any information related to pricing, new products, promotions, sales
     strategies, or marketing information.

(c)  All training material, oral or written.

(d)  Any information on products, sales techniques or methods.

(e)  Financial information of any kind and nature relating to RETAINER or
     Related Companies.

(f)  Group or Trader Information, notwithstanding it source of origin, i.e.,
     telephone directory, social connection, or newspapers, after the group,
     person or trader engages in a relationship with RETAINER or Related
     Companies.

(g)  Any information related to software and computer systems.

(h)  Any information relating to the business or corporate development
     strategies of RETAINER or Related Companies.

     The OFFICER agrees to keep confidential all trade secrets and confidential
information for the benefit of RETAINER and he shall not use nor disclose any of
such information for any purpose other than RETAINER's business.

                                       5
                 [Retention Agreement Chief Executive Officer]
<PAGE>

SECTION 08.     The OFFICER hereby acknowledges that all registrations, reports,
----------
notes, compilations and other processed materials, as well as the respective
copies or reproductions, in connection with the operations, activities or
businesses of RETAINER, its controlled companies, affiliates and subsidiaries,
made or received by the OFFICER during the validity of this Agreement with
RETAINER are and shall be exclusively owned by the latter, and the OFFICER shall
keep those materials always under his safekeeping and control and, upon
termination of his term of office, such materials shall be promptly returned to
RETAINER.

SECTION 09.     From the date hereof until eighteen (18) months following the
----------
date on which the OFFICER ceases to be an officer of RETAINER (the "Restriction
Period"), neither OFFICER, nor any of his relatives up to second degree or any
entity with respect to which OFFICER, directly or indirectly, individually or
collectively, owns more than (thirty percent) 30% of the total voting securities
or other ownership interests (each a "Subsidiary"), shall:

     (a)   directly, or indirectly through any other individual or Subsidiary:
(i) own, (ii) manage (as an employee, consultant or otherwise), (iii) operate,
(iv) render services to, (v) become interested in or associated with, (vi) join
in, (vii) control (by virtue of ownership of voting securities, contract or
otherwise), (viii) participate in, or (ix) otherwise carry on any Competing
Business as hereinafter defined. For purposes of this Agreement, a "Competing
Business" shall include any entity, person, individual or business other than
RETAINER engaged in the activity of providing internet connectivity and value
added services, including, but not limited to, internet access, domain
registration, web hosting, web site management, intranet, extranet, electronic
commerce and virtual private network services in Brazil;

     (b)   directly, or indirectly through any other individual or Subsidiary,
enter into any employment agreement or contractual relationship of any nature
with or related to any Competing Business, except with any entity of RETAINER;

     (c)   directly, or indirectly through any other individual or Subsidiary,
solicit, entice, persuade or induce any Employee, as hereinafter defined, to:
(i) terminate or refrain from renewing or extending his or her employment with
RETAINER; or (ii) enter into any contractual relationship with any Competing
Business. For purposes of this Agreement, "Employee" shall mean and include all
persons employed by Dialdata on the date hereof or at the time of taking of any
of the actions prohibited by this Section 1(c);

                                       6
                 [Retention Agreement Chief Executive Officer]
<PAGE>

     (d)   directly, or indirectly through any other individual or Subsidiary,
solicit, entice, or induce any Customer or Supplier, as hereinafter defined, to
terminate, reduce or refrain from renewing or extending its contractual or other
relationship with RETAINER, to become a customer of or to enter into any
contractual or other relationship with any Competing Business. For purposes of
this Agreement, "Customer" shall mean and include all individuals or entities
that acquire services from RETAINER during the Restriction Period, and
"Supplier" shall mean and include all entities from whom Dialdata purchased or
leased services, goods or other supplies during the Restriction Period.

     Sole Paragraph.    In case of early termination of this Agreement by the
RETAINER the non competition obligation set forth in this Section may only be
valid and in full force, if so requested by RETAINER and against monthly payment
to the OFFICER in an amount equal to fifty (50%) percent of the balance of the
base consideration (and not any bonus) set forth in Section 03, payable in
monthly installments.

SECTION 10.     The OFFICER hereby represents that he is not under any
----------
confidentiality obligation or other commitments that may conflict with this
Agreement or which may limit or restrict in any way the scope of his activity or
which is incompatible with the obligations assumed herein.

SECTION 11.     This agreement is irrevocable, binding upon the parties hereto,
----------
and may only be amended by written document signed and executed on the basis of
mutual consent.

SECTION 12.     This Agreement shall supersede all other prior agreements,
----------
whether written or oral, in connection with the rendering of the services or the
conditions herein set forth. This agreement does not amend or replace any other
agreement made by and between the OFFICER, on one side, and V-I-A Brazil and/or
its controlling quotaholders and/or RETAINER on the other side, including, but
not limited to, shareholders' agreements and non-competition and confidentiality
agreements, which shall be valid and continue in full force and effect for their
respective period of validity.

                                       7
                 [Retention Agreement Chief Executive Officer]
<PAGE>

SECTION 13.     This Agreement is subject to specific performance as set forth
----------
in articles 642 et seq of the Brazilian Civil Code of Procedure.

SECTION 14.     The parties hereby elect the Courts sitting in the City of Sao
----------
Paulo, State of Sao Paulo to settle any disputes arising out of this agreement,
with express waiver of any other however privileged.

                                  * * * * * *

                                       8
                 [Retention Agreement Chief Executive Officer]
<PAGE>

IN WITNESS WHEREOF, the parties hereto cause this Agreement in two counterparts
of same content and for the same purpose, in the presence of the 2 (two)
undersigned witnesses.

                           Sao Paulo, December 29, 1998.

                                   DIALDATA S.A. INTERNET SYSTEMS

                                   By:  /s/ Rubens Mau
                                       -------------------------------------
                                       Name: Rubens Mau
                                            --------------------------------
                                       Title: Chief Operation Officer
                                             -------------------------------

                                   By:  /s/ Joaquim Jose Rodrigues Torres
                                       -------------------------------------
                                        Name: Joaquim Jose Rodrigues Torres
                                             -------------------------------
                                        Title: Chief Technology Officer
                                              ------------------------------

                                   /s/ ANTONIO TAVARES
                                   -----------------------------------------
                                        ANTONIO ALBERTO VALENTE TAVARES

Witnesses:

1. /s/ DANILO MARQUES DIAS LOMBARDI
  ----------------------------------
Name:
CPF:

2. /s/ SERGIO BRONSTEIN
  ----------------------------------
Name:
CPF:

                                       9
                 [Retention Agreement Chief Executive Officer]

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