I've started watching some Florida State full games. I've seen 3 or 4, they play a lot of full court press zone defense which means they end up match crossing all around the court. When they play man they also switch a lot. Many times Vassell would end up in the post guarding a guy much bigger than him and he more than holds his own with those longs arms, quickness and leaping ability. Per 36 minutes he averaged only 0.1 less rebounds than Williams and only 0.5 less blocks. So, if there's such a length difference, as you seem to believe, it isn't reflected on the numbers.
The way I see it, Vassell > Williams in terms of defense. Whatever advantage Patrick might have banging down low, Vassell more than makes up for it with his great perimeter defense and crisp defensive rotations (Williams is very shaky on these two areas). What makes Williams a better prospect than Vassell is his offensive potential.