Really tough matchup, I actually really like both these bowlers.
Overall, I'll go for Vaas, mainly on the grounds of consistent effort through thick or thin. Chronically underrated, a very wily bowler who frequently plays on surfaces not that helpful. I think he's given a great deal of respect by batsmen around the world, and he's definitely earned it.
When on song, the Shoaib is an excitement machine, and there are few sights as satisfying as when he's skittled somebody's stumps out of the ground. But his attitude has always been in question, and his illegal substance use doesn't help, although I don't view it in quite the same condemning way as match-fixing. (To be honest, I'm not sure his substance abuse would have changed my vote either way.)