In order to do it in the best way, we are still waiting shirokun for the bass track...
Anyways, if both guitars were waiting for bass track, I think you can start to record your part without bass track. If you really need it, you might be able to do it using the "original" song.
About vocals, I don't know but it would be better that all instruments tracks have to be recorded.
EDIT: As shirokun doesn't answer in this topic yet, I'm going to PM him.