I understand that you see error starting after vBET is installed. But as you see vBET has nothing to do with it, because error still appears when vBET is disabled. So also some other changes had to be done on your forum. Maybe some other mod was also added, or some configuration changed.

First please determine who is guilty. So disable one by one all plugins which are working with hook 'postbit_display_complete' - each time check do you still have error. When you determine who is guilty then please contact author of mod which makes the mess. If you still have some questions - please ask