They are completely separate software packages and you would need to buy vBGallery.
Glowingal vbgallery is essentially a vbulletin gallery that has some better use of the integration since it is an addon to vbulletin and does not work without it. It's templates and administration are all in the admin panel. Now Photopost of course sports over twice the features of vBGallery so thats the main difference is users tend to like the integration of vBGallery better since it is a
VB addon but for it's price and limited code structure it sacrifices features to its big cousin PhotoPost.