There's no need to switch to vBgallery from PhotoPost Pro. PhotoPost integrates very nicely with vBulletin, including the login system, displaying galleries and photos on your vbulletin forum index page, and using your vbulletin header/footer and stylesheet. vbgallery is a stripped down version of Pro - you will lose hundreds of features if you switch. The only advantage vbgallery has over pro in terms of a vb integration is that its admin panel is part of vb's admin panel, and it uses vbulletin's template system.
That said, you will need to configure PhotoPost Pro to use the vbulletin integration code instead of your existing forum integration code. In the "All Options" panel in PhotoPost, change "User registration system" to "vb3" without the quotes, and specify the appropriate forum database table prefix for your vbulletin tables (if there is a prefix).
config-inc.php needs to be configured for your vbulletin database (if it changed), and you need to enter your vbulletin license number in config-inc.php.
You'll need to go to the edit integration tab in photopost, and specify the correct information there to integrate with vBulletin.
If you still want to switch to vbgallery from Pro for some reason, then you will have to do all of the above first, then install vbgallery, then import all of your photos and categories into vbgallery: