Hmmm....Vandy couldn't put it in writing because...? Sounds a lil' fishy.
I agree w/Remi - go for what you want to do more, but realize that if Vandy cannot commit to you, you risk paying ALL the tuition there vs NONE of the tuition at Nebraska (which I am assuming made you the offer in writing).
Get Vandy to put pen to paper, and go for what you love!
dc