It's my understanding that if an instrument approach includes a VDP, you
should not descend below the MDA prior to reaching this point. If
you're not equipped to identify the VDP you should fly the approach as
if no VDP is provided.

I've been told you're not _authorized_ to descend prior the VDP but that
makes little sense. If you fly the approach without being able to
identify the VDP you may well descend prior to the VDP without even
realizing it and that would still be acceptable.