Why would you do a clean install of the gallery. There are several threads on "switch integration" on the forums here. In none of them are you importing gallery content so correct a forum import script is only going to import what it was written for meaning your vb
content. For you not to lose anything you need to use your existing photopost folder and database tables.
here are the steps as I see it
1. You install your new forum
2. You use your xenforo import script to do your forum import
3. You delete or clear out your photopost usergroups table.
4. You reinstall photopost over top of the existing gallery database tables to integrate with xenforo.
5. There is a script in the Pro import forum called userfix.php you can download its in the realign userid's when switching to vb
thread. You can use that to realign userids that will be messed up when you use forum import scripts to import to a new forum they change userids
Once all thats done you should be able to login with your xenforo admin and set permissions etc