Right but I do not beleive you let anyone know you where running such an older version is what I am saying. You can do what you want to do but its a long drawn out process. I take it your old gallery is installed with a vb3 gallery already.
1. You need to upgrade your gallery to PhotoPost Pro 7 series download that already supports vbulletin 4 so upgrading is going to have your gallery working with your vbulletin you just upgraded to version 4 from 3.
2. If you want to switch to vbgallery you can do that from the 7 series by installing vbGallery and then using the import script utility in the download of vbGallery under the do not upload folders the filename I meantioned to import Pro content to vbGallery.
You do not need to switch to vbGallery if you want to continue running Pro. Its all up to you what level of work you want to do to run what.