The problem I had with the Vista bakup and dual booting is that Vista included XP in the backup image and did not allow me to only image Vista. I had mine on 2 different hard drives, I had to use a boot manager to remove the XP boot before I could get Vista to repair its boot sector to a sigle boot.

If you are concerned about the state of your current Vista partition, you can use a partitioin program, like partition magic or similar, to to non destructive partition changes. Just make sure the program you use will do Non Destructive changes, not all are capable.