Gloucester big-name summer signing shone on a ground he would have been familiar with during his days as a Bradford Bulls player.
"I always love playing here - in both codes. I didn't think I was going to get that many tries, "he said after scoring in the 36th minute and then crossing four times in the final quarter, "I was just going out there and concentrating on the way I want to play."
Gloucester are pooled with Magners League champions the Ospreys, 1999 European champions Ulster and Bourgoin, whose European pedigree stretches back to their 1997 Challenge Cup success.
Vainikolo's performance would have been noted.
Elsewhere, there was a shock result for Heineken Cup holders London Wasps who were beaten 29-19 on their home patch of Adams Park.
Ex-Munster boss Alan Gaffney masterminded the victory and said: "I thought we played pretty well. We set out with a game plan and our guys adhered to it. We played with a lot of composure and a lot of pace and played the game we wanted to play.
There was better news for the other former Heineken Cup winners. Bath, champions in 1998, beat Worcester 29-15 to spoil Mike Ruddock's first game in charge of the Warriors.
Two-time champions Leicester made short work of Bristol at the Memorial Ground. Even without a large quota of their pack who are on World Cup duty, the Tigers won 13-26.