As you're tracking these numbers, you might want to also look at Duke's numbers - there's surely more candidates choosing between these 2 than any other, and I'd have to imagine that Duke tends to win that contest, opening up spots on the Vandy waitlist. This forum suggests everyone should always choose Duke over Vandy with similar $$$, but I was surprised at ASW to see about 150 of 200 people there were trying to make the exact same decision as I was between the exact same 2 schools (I clearly chose the latter). you might see a trend!
The thing I have noticed about Vandy is that, in general, if you got into Vandy you got into at least 1 t14. Since they give about 70% of admitted students schollys, it comes down to $$ at Vandy v a t14 which is a tough choice. Many of the admitted no action students have comparable offers elsewhere.
Also my data is skewed because people don't always update their LSN profiles. I have seen at least one person who was accepted at Harvard and put the attending A next to Harvard but didn't put the withdraw W next to Vandy, so for my data I had to count her as still on the wait list at vandy (I didn't want to make any assumptions)