This is called 'bridge'. There are some ready 'bridges' among softwares. You should search if VB has a bridge for this photo gallery you are using.
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The code is usually not too bad to modify. When bridging applications, you make one the master application (VBulletin in your case maybe), and disallow logging in through the other. When they click to log in, they are redirect to the vBulletin register/log in page and they handle it all there. Then your vBulletin code would need to be modified to pass a few extra session variables to let the client know they are logged in on the other site as well. If you are using well know programs, it isn't too hard to get it all done or find someone who can.
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I remember doing this over 10 years ago with a phpBB forum and another script. It worked great but the problem would be if someone updated or had their account deleted, you would have to have code to update the other database as well (assuming both scripts do not share the same DB).