Please give us exact version of used vBET, vBulletin, vBSEO (if used).
Also please tell us which way you are renaming discussion and are you doing it on normal or on translated page?
I cannot reproduce it on our forums so I will need little help on your side
OK - I checked source and it is caused by some plugin using hook "postbit_display_complete". vBET has 2 plugins using this hook. Can you please determine which one causes troubles on your forum? You can simply do it by disabling one and check does issue still exists. If yes then enable it and disable other one. If still exist then try with booth disabled. If it still exists - then most probably it is not vBET fault - try do disable whole vBET and check.
You will disable plugin by Admin CP-> Plugins & Products -> Plugin Manager
You will disable whole mod by Admin CP-> Plugins & Products -> Manage Products
Please let me know about results. If you PM me access details I will check it myself
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