Paupio Namai used to be called 'Uzupio Hostel' but changed the name about a year ago. Not sure what else has changed (probably not much)...
Okay, admittedly this place looks a bit dodgy, but really it's not. I stayed here a year ago and had an excellent time. In fact, it was from here I think I fell in love the city. It really captures the essence of Vilnius for me. When I was there, there was no way you would characterize this place as anything but a hostel. But now it looks like the prices have gone up and maybe that's why it isn't listed as one. Expect old-world hostel standard when you arrive. It's all charm, baby...