The former Bradford Bulls rugby league star has not played since December 29, having flown back to New Zealand following the death of his father.
But Gloucester will field Vainikolo for the Ulster game, which comes just 24 hours before he meets up with England's RBS 6 Nations squad at Twickenham.
Vainikolo said: "I have been fortunate to have received a great deal of support, but I am really happy to be back, be around the boys again and thinking about rugby.
"It is great I have an opportunity this weekend, and I want to go out and repay the faith people have in me.
"There is a job to do on Sunday and we must make sure we achieve it."
With the juggernaut wing absent, Gloucester lost their last two games - against Guinness Premiership opponents Bath and Heineken Cup rivals the Ospreys.
Victory on Sunday, though, would see the Premiership leaders secure a home quarter-final.
Gloucester head coach Dean Ryan said: "Lesley has obviously been through a very tough time.
"But he is a very positive guy and I am sure he will channel it into a performance.
"He handles himself very well and is really keen to get back on to the pitch with us."
Vainikolo is among six changes from the side thumped 32-15 by the Ospreys in Swansea last Saturday, as Ryan also hands starts to fly-half Chris Paterson, scrum-half Rory Lawson, prop Nick Wood, hooker Olivier Azam and flanker Alasdair Strokosch.
Peter Buxton, meanwhile, moves from back-row to second-row duties as replacement for broken arm victim Alex Brown.