VBB is a process code. Not sure exactly what it means but it is something to do with the factory / packaging / ESD.
It has nothing to do with functionality what so ever!
Lots of oher Amiga chips (Denise, Paula, Budgie etc.) made at the same time also have VBB on them.
I've got 2-Mbyte Agnus chips without VBB printed on them and I have 1-Mbyte ones with VBB printed on.