Here this goes lets go through this.
1. Yes you can host the forum and gallery on different servers and integrate the user login authentication if the forum server allows external mysql connections.
**NOTE. You will have to login at each site since usually in this scenerio the domain name is also different.
2. You may not use the vbulletin enhanced integration if the forum is on another server since we require the server path to the vbulletin global.php file among other files all of which will not exist on the server photopost is on.