i'll probably only be going to the one on the 15th. is there much variation in the vendors from week to week?
There will be a little variation, but not a ton. I believe most people are signed up to be there all three weeks. Check out the vendor list on the site and they should say on their websites/twitters when they will be there.
Cinnamon Snail will be there all three weeks for sure, so you'll be able to get a pumpkin white chocolate donut no matter what!