VFC tends to do either licensed releases, or unlicensed. When they do licensed releases, like an official KAC L403A1 replica, they tend to make sure everything is above board. They get the rights to MLOK via KAC, for example, as they're reproducing KAC's design that in turn includes Magpul's MLOK IP.
When VFC wants to do something completely unlicensed, they go to one of their shell companies, like Maruyama, APFG, or Star Arms. That way they can do B&T or Sig products without incurring the relevant costs or design oversight. For example, VFC P320s via Stark Arms or Parabellum run much better overall than the officially licensed Sigair P320s because Sig demanded they be designed for CO2 first and foremost as military/law enforcement training aids and Airsoft toys distant second.
Generally KAC is not gunna want its officially licensed replica to have stolen IP from other companies, so you tend to get basic A2 grips and the like on them. Overall it tends to be pretty all-or-nothing in terms of replicas.
That's before you get into companies that just hold licenses and outsource to companies like VFC, such as Umarex or Cybergun.