Are all the old songs in BK? because I really want Disco Prince. And you unlock the songs in VS. Mode right?

Only a few are. Disco Prince isn't one of them though.
Off the top of my head:
WANDA WANDA
You Are Smart
Katamari Holiday
Houston
The Royal Academy of Katamari
Angel Gifts

But with the 360's custom soundtracks you can technically listen to all of the old songs over the new ones.