2. Not sure exactly what you mean, but if you mean keeping your old XP Backup and new Vista Backup on the same drive - Yes, in separate folders.
3. If you mean as in my answer 2. then I would say Yes - but Not if you mean running the two OS's side by side, I think that would bring complications.
You can of course run both systems on one drive (your main C:) by Dual booting. Simply Google for How to .......