Well all this depends on xenforo and what import scripts your using. While we can not officially support switching an integration or adverse situations because of that I can lay out the steps and we even have a script that might realign the userid's for you but that of course totally depends on every single user being imported with the correct username into that new forum.
1. import your vb
forum into xenforo using any import scripts xenforo has for such an action.
2. Delete the pp_usergroups and cp_usergroups tables from your databases for our products as you will be reinstalling the apps and importing new usergroups.
3. Reinstall the gallery and classifieds to integrate with xenforo make sure to properly input any xenforo info like globalSalt table prefixes or cookie prefixes
4. Run the userfix script located in our import forum to realign the userids which will change due to the forum import.