So if Bitdefender develops the engine, what does the anti-virus vendor do? Only develop the input (definitions)?
In a nutshell, most anti-virus vendors that are licensing the Bitdefender engine extend it with their own engine to improve their detection rate. For example, they support more exotic file formats and binary packers, or fancy heuristics, etc. Essentially this means they have the full attack surface from Bitdefender plus their own attack surface...