[Deceptor]

vB Optimise doesn't directly tie in with usergroups anywhere. The only cache it will potentially handle is Phrases (if you have Phrase Cache on and haven't disabled phrase cache in the ACP), and also handle custom datastore items. Nothing is actually "set" on this page unless those phrases/datastore items aren't already cached.

If you change the usergroup all the time will the error come up every time, or just some of the times? Also try removing the affected PHP line or comment it out and see if it rectifies the issue, as said before the function isn't required.