Improved vietnamese support for landlines and mobile

Hi, Looks like the existing Vietnam support was limited to certain NDCs and mobile numbers.  This PR expands that support to all listed NDCs as per http://en.wikipedia.org/wiki/Telephone_numbers_in_Vietnam

NOTE: This changes existing behavior as demonstrated below with the Hanoi spec. Previously it incorrectly listed area code `4`'s subscriber length as `7` digits instead of `8` digits ([see wikipedia](http://en.wikipedia.org/wiki/Telephone_numbers_in_Vietnam#Area_code_4)).