Gloucester's former rugby league heavyweight could be handed his Red Rose debut if he steps from the bench on Saturday and Williams said: "It's going to be David versus Goliath if we face each other, but I've got to deal with it and bring him down by any means possible, as long as it's within the law.
Williams may give away seven inches in height and over four stone in weight to Vainikolo but has made a living out of proving he can knock the big men from their stride.
"I've played against plenty of big wingers like the Tuilagi brothers and France's Aurelien Rougerie. They're all monsters compared to me," he said.
"I've learned the hard way that you have to tackle these guys around their legs. If I try to tackle him high then he'll probably chuck me into the stand. Sometimes it's easier playing the big men, you know what you're going to get rather than someone like Jason Robinson who can step past you in a heartbeat."
Williams revealed he will take a leaf from another apparent miss-match when South Africa wing James Small stopped All Black monster Jonah Lomu in his tracks in the 1995 Rugby World Cup Final.
He said: "I'll have to close him down very quickly and not give him the space to run at me. Look at what Small did to Lomu. He was on him before he even got the ball - and it worked."
The Ospreys wizard is among the most prolific finishers in world rugby with 35 tries in 51 Tests for Wales, and was the second highest try scorer at the World Cup. Now he is ready to beat the odds yet again.
Williams scorched over for the winner against England in 2005 that set Wales on their way to a Grand Slam. Now he needs just one more try to become Wales's top finisher in the RBS Six Nations.
He said: "I don't want to wait around on the wing for the ball. I feel happier in myself when I look for work and get my hands on the ball."