One can argue that for a user it doesn't matter if it's ffmpeg, VLC or something else that's buggy. A user is installing VLC, has probably no knowledge of what ffmpeg is, and it's the use of VLC that exposes the user to the security bug.
As such, it's VLC responsibility to not ship a product that may harm a user's computer, even if the error is not in their code.
As such, it's VLC responsibility to not ship a product that may harm a user's computer, even if the error is not in their code.