bwolcott,
PhotoPost vBGallery is another gallery solution offered by PhotoPost. The major difference with vBGallery is that it will only work with vBulletin (as opposed to PhotoPost PHP Pro that will work either stand-alone or with many other forum type) and that vBGallery is deedply integrated with vBulletin (as opposed to PhotoPost PHP Pro which has it's own admin' interface).
Feature wise there are some items in PHP Pro which are not in vBGallery and some that are in vBGallery but in PHP Pro. This is due mainly to the fact that the two products are developed seperately and do not share any code.
You can find a comparison of the two products
here. If you're already a PhotoPost PHP Pro customer then most likely you won't find any overwhelming reason to consider switching to PhotoPost vBGallery but if you're a new customer looking for a vBulletin gallery solution then vBGallery makes for an extremely nice alternative to PP Pro.
You'll find that both products have some pretty ardent fans but in the end either product will fill the needs of most vBulletin communities.