From experience I know that the only way to get off the waitlist at Vandy is to make it in clear to admissions in no uncertain terms that you will accept if moved off the waitlist. I am sure that the waitlist letter is the same this year as in prior years where it states that demonstrated interest is now the dominant factor in admission decisions off the waitlist. IMO, it is a crummy system and reflects poorly on the school (as it does for others school who do it, too)