Yeah, I ran it side-by-side with my PhotoPost installation for quite a while until I was ready to switch over. Photopost was in /photopost and Gallery was in /gallery. Install Gallery and run the import script so that you have data to work with. Then when you're ready to switch, re-do the import so it's all up-to-date.
You can install it into its own DB, or into the vBulletin DB. (I've done it both ways.)
Once you switch, you may be interested in this as well:
http://www.vbadvanced.com/forum/showthread.php?t=5931