doraj - Enable debug mode on your forum, then check the debug messages in the bottom drop down menu to see if vB Optimise is successfully pulling from cache, or continually trying to set the cache from the database.
Logicuk - My bad, I will definately take a look again
Leo Brazil - What cache method do you have set? The hook there is designed to pull a method from vB Optimise and it seems that method is not being created and reverting back to the default vB Optimise class.