as for places to go - do you like coctail bars, or run down pubs, or dance clubs...?
Some good places in Bratislava are Verne on Hviezdoslavovo Square, my favorite, due to many a long night in my university years is Cafe Galeria on Panska, then there is Praná Bata-all are more pub-like.
For cocktails - Paparazzi, ....
I just found a nightlife guide online which looks good:http://www.bratislavahotels.com/bratislava-nightlife-clubs-disco
EXCEPT: Stay away from the Slovak Restaurant, Harley Davidson, and Coyote Ugly. Good thing about B'lava is that it's so compact, so anywhere you walk in downtown you're gonna find pubs and restaurants. It seems I'm on an opposite schedule - will be in Vienna on 7th, coming to B'lava on 8th in the evening until the 10th but you'll have a grand ole time.
In Vienna...well, again, depends on what you like. For some Austrian pubfood you can go to any Centimeter pub (there's a few- one in each district) - where they have sandwiches and sausages by meters. And good beer on tap. For cocktails - there's a good sort of 'ethno' bar with interesting world but dance music called Gazelle off of Mariahilfestrasse. There's also a Bar Lutz nearby, sort of shi-shi (sp?) though. If you want a dirty ole student grunge bar, Donau is your place (dark and smoked up, you can even smoke wacky tobacky there, if you're into that) - also off of Mariahilfestrasse. For a late night, you'll have to hit Bermuda Triangle in the first district - bunch of places there in one heap, just walking around will do.
.....anyhoooo, feel free to ask any questions that would come to mind. are you planning to do the whole sightseeing thing, or just sort of walk around and see what happens? some good music in both towns - babylon club in bratislava (ask for YMCA) has decent live jazz, in vienna there's a bunch of stuff. you can look stuff up on
www.falter.at - though i think that's in german only. but you should be able to navigate around nonetheless...