I used Greyface patch 2.4, which I believe is new since my last playthrough a few years ago, and potentially related to the new GOG versions. So yes, Greyface may have tweaked to allow a previously unable (but implemented) item to drop.
There is no specific quest for the Vampire Crypt on the Dark path. You can enter as normal, but the only quest linked to the dungeon is one of the "adviser" quests linked to the Light path, which you obviously don't get on the Dark path. It's possible that it only drops in the dungeon when you're on the Dark path (and therefore no specific quest), but it's also possible this is a random drop that just happened to drop for me for the first time on this Dark playthrough. Or it's possible that it now drops 100% with this patch. I'm keen for others to test this!