Vainikolo's switch to rugby union with Gloucester is widely expected
Wakefield: (4) 4
Bradford: (10) 14
Lesley Vainikolo scored two tries as Bradford edged out Wakefield to reach the Challenge Cup quarter-finals
Tries: Vainikolo (2), Cook
He collected Paul Deacon's kick to score, despite suspicions of a knock-on and added a second, racing clear after receiving a pass from James Evans.
Wakefield reduced the deficit to 10-4 by half-time when Jamie Rooney fed Paul White, who scored in the corner.
The Wildcats kept up their resistance after the break but a late score from Matt Cook sealed the win for the Bulls.
A week ago Bradford lost to Leeds after a controversial decision by referee Steve Ganson and video referee Ashley Klein, but this time the video referee ruled in their favour to award Vainikolo his first try and set the Bulls on their way.
Wakefield fought back after Vainikolo's second and soon got on the board through White's try.
The home side continued to press forward after the break, and things looked to be getting even worse for Bradford when Chris Feather was taken off with a suspected broken leg.
Deacon then dragged his attempt at a drop-goal wide before Cook calmed the nerves by collecting a loose ball and touching down late on.
Bradford boss Steve McNamara on Vainikolo, who has been linked with a move to union side Gloucester:
"Who's said he's leaving? There's always the possibility that anyone can come and go.
"Les was absolutely outstanding. Never mind the two tries, his contribution was probably the best I have seen since I have been here.
"He has been fantastic for us and still is fantastic for us."
Wakefield boss John Kear on the first try:
"I thought it was a joke. The laws of the game state if the ball travels forward it is a knock-on.
"I was mystified to tell the truth, but not surprised. I thought there were a plethora of knock-ons.
"I thought we fronted up and had a dig. It was not the prettiest of games because of the rain but we got back to doing the basic processes pretty well.
"Obviously it was difficult to create chances, but both teams found that."
Wakefield: Rooney, Demetriou, George, Atkins, White, P. March, Obst, Moore, D. March, MacGillivray, Henderson, Catic, Ferres.
Replacements: Blaymire, Jeffries, Golden, Bibey.
Bradford: Platt, McAvoy, B. Harris, Evans, Vainikolo, I. Harris, Deacon, Burgess, Henderson, Lynch, Solomona, Morrison, Langley.
Replacements: St Hilaire, Cook, McKenna, Feather.
Referee: I Smith (Oldham).