Question: Pentatonix, the five-member American a cappella group lost their vocalist Avi Kaplan in 2017 at the same time as their tribute to which group received over 250 million views?

Context articles: Pentatonix discography
Pentatonix
Pentatonix (album)
Avi Kaplan
Northwestern THUNK a cappella
Metro Vocal Group
Can't Sleep Love
Pentatonix World Tour 2016
Kirstin Maldonado
A Pentatonix Christmas

American a cappella group Pentatonix has released four studio albums, four extended plays (EPs), eight singles and twenty six music videos.
 The group won the third season of "The Sing-Off" in 2011, and subsequently received a recording contract with Sony.
 In June 2012, Pentatonix released their first extended play, "PTX, Volume 1" through Sony-owned independent record label Madison Gate Records.
 The EP debuted at number 14 on the US "Billboard" 200, selling 18,401 copies in its first week of sales.
 A Christmas EP, "PTXmas", was released in November 2012.
 It was re-released in November 2013 with one of its new tracks, "Little Drummer Boy", debuting at number 13 on the US "Billboard" Hot 100 and charting in other countries such as Austria, Canada and New Zealand.
 The EP became the sixth highest-selling Christmas album of 2013, selling over 168,000 copies.
 In November 2013, Pentatonix released their third EP, "PTX, Vol.
 II" which debuted at number 10 on the "Billboard" 200, selling 31,000 copies in its first week of sales.
Pentatonix (abbreviated PTX) is a five-member American a cappella group from Arlington, Texas, consisting of vocalists Scott Hoying, Mitch Grassi, Kirstin Maldonado, Avi Kaplan, and Kevin Olusola.
 Characterized by their pop-style arrangements with vocal harmonies, basslines, riffing, percussion and beatboxing, they are widely known for their covers, mostly of modern pop works or Christmas songs, sometimes in the form of medleys, along with original material.
 Pentatonix formed in 2011 and subsequently won the third season of NBC's "The Sing-Off," receiving $200,000 and a recording contract with Sony Music.
 When Sony's Epic Records dropped the group after "The Sing-Off", the group formed its YouTube channel, distributing its music through Madison Gate Records, a label owned by Sony Pictures.
 With over 13 million subscribers and 2.4 billion views, Pentatonix's YouTube channel is currently the 50th most-subscribed YouTube channel.
 The group's video tribute to Daft Punk had received over 250 million views as of mid 2017.
Pentatonix is the fourth studio album by American a cappella group Pentatonix, which was released on October 16, 2015.
Avriel Benjamin "Avi" Kaplan is an American singer and songwriter, most well known as a past member of the a cappella group Pentatonix.
 He is known for his bass voice, and often acted as the vocal bass of Pentatonix.
 In May 2017, Kaplan announced that he was leaving Pentatonix following their upcoming tour.
THUNK a cappella is a 16-member co-ed collegiate a cappella group from Northwestern University in Evanston, Illinois.
 Founded in 1993, the group was originally called Melodious Thunk, created after the giant in American jazz music, Thelonious Monk, but the name was changed simply to THUNK because of legal formalities.
 THUNK is Northwestern University's oldest a cappella group on campus.
 International Championship of Collegiate A Cappella (ICCA) award-winners, and regularly featured on various a cappella media like the InsideAcappella video blogs and The A Cappella Blog, THUNK is entirely student directed and produced.
 They have two to three shows each year and travel to Cape Town, South Africa every other year to provide music therapy and musical education outreach programs.
 In addition to on-campus performances each year, THUNK performs at major events in the Chicago area.
 Some of THUNK’s notable performance venues include Northwestern University’s President’s Convocation, Wrigley Field, Westminster Abbey in London, England, The Museum of Contemporary Art in Chicago, the Royal National Theatre in London, England, and the Amphitheater at the Victoria & Alfred Waterfront in Cape Town.
Metro Vocal Group is an American a cappella group based in Hong Kong.
 Metro’s repertoire is a mix of traditional music fused with rock and pop as well as covers of popular songs worldwide.
 In an a cappella fashion, Metro uses their voices to imitate percussion, trumpets, guitars and miscellaneous sound effects as well full melodies with harmony.
 Metro is best remembered for their music video covering "Boundless Ocean, Vast Skies" (海闊天空), a song made famous by Hong Kong rock group Beyond, and their cover of Joey Yung's "My Pride" (我的驕傲).
 The group has since received over 12 million views worldwide on video sharing websites like YouTube and Youku.
 Since that time, Metro has released two albums, "Music for a Metropolis" and "No Borders", an all Chinese album, and has received acclaim throughout Greater China.
"Can't Sleep Love" is a song recorded by American a cappella group Pentatonix from their eponymous fourth studio album, "Pentatonix" (2015).
 It was released as the lead single from the album via RCA Records on September 4, 2015, and is Pentatonix's first original track.
 It was written by all members of Pentatonix with Elof Loelv, Kevin Figueiredo, Teddy Peña and William Wells, and was produced by the band.
 A second version of the song, featuring American musician Tink, was released on September 18, 2015.
Pentatonix World Tour 2016 (titled as Pentatonix World Tour 2017 for the shows in 2017) was the second headlining concert tour by American a cappella group Pentatonix to promote their eponymous album.
 The tour began on April 2, 2016, in Chiba at Makuhari Messe, and concluded on September 3, 2017 in Essex Junction at Champlain Valley Exposition.
Kirstin Taylor "Kirstie" Maldonado (born May 16, 1992), is an American singer and songwriter, and a member of the a cappella group Pentatonix.
 Together with Pentatonix, Maldonado has released seven studio albums, won three Grammy Awards, and sold over six million albums.
A Pentatonix Christmas is the fifth studio album by American a cappella group Pentatonix.
 It is also their second full-length holiday album since "That's Christmas to Me" in 2014.
 Featuring two new original songs, "Good to Be Bad" and "The Christmas Sing-Along", "A Pentatonix Christmas" debuted on the "Billboard" 200 at number three with 52,000 albums sold in its first week, and later peaked at number one, selling 206,000 units in its best week.
 "A Pentatonix Christmas" marks as their second number one album on the "Billboard" 200 after "Pentatonix".
 The album also debuted atop the "Billboard" Holiday Albums chart, their second number one on that chart after "That's Christmas to Me".