You need to upgrade your older Photopost Pro 4.8e to the latest version by downloading the source code and upgrading based on the instructions in the upgrades text in the documentations folder within the download. The version your running currently was released before vbulletin 3.5 series or the later 3.6 series came out thus upgrading will ensure you have the proper software versions to work with one another.
Now when you switched your integration did you follow some of the tutorials here? I think if you take a look at your usergroups table of photopost via phpmyadmin you will probally still see your old ubb groups in Photpost. What you want to do is after updating your software delete all usergroups except administrators in that table and then edit the entry and change the groupid field from 1 to a 6
You should then be able to login to admin and import your vb