Riga is fairly popular because of Ryanair. There are a couple of hostels in Vilnius - the one where we got our hosteling cards was fairly busy when we stopped in, but we lived there, so I didn't really know the hostel scene. For night-life, check out
www.inyourpocket.com - they have excellent city guides and the reviews are accurate in our experience. We relied on the restaurant reviews pretty heavily and agreed 90% of the time. In particular, if they say the food is bad, it is.
Give me an idea of your interests, and I can give you a more tailored list of suggestions, but be sure to check out the Occupation Museum (Riga) and the KGB Museum (Vilnius). Then go for comfort food. You'll need it.
I've given lists of my favourite things in Vilnius in other posts in the forum - don't have time to rehash them now, but check them out and that'll give you some ideas. I'll try to give you specifics if you let me know your particular interests.
Enjoy, it's beautiful. Can't wait to go back.