You will not need to update PhotoPost everytime you update Vbulletin. Basically in the past years there have only been two instances usually where PhotoPost has had to be updated if you upgrade vbulletin. Vbulletin has put out many upgrades in the past 4 or 5 years.
1. When VB3 was released people running older Photopost's that where compatible with vb2 only and wanted to upgrade the forum would have had to update Photopost also.
2. With the release of VB
3.5X series a user would need to make sure to upgrade to minimum PhotoPost 5.21
Basically I recommend separate databases but there are merits on both sides of the arguement. There are threads on here if you want to do a search. Do what is right for your hosting plan etc. If you have to pay for an extra database do not do it. Run them in the same database.