Weird. I was born in SC and lived there into my early 30s. Born in the midlands, later moved to the upstate and then in the low country for a few years. Ate at multiple BBQ joints everywhere I lived. Never came across vinegar-based, except for one little out of the way joint in Pickens that advertised itself as NC BBQ. The upstate was definitely more tomato-based, but there were still some with mustard. When I lived in Charleston, most of the places I went were mustard-based. I didn't even think whether there would be mustard based or not.
It would have struck me as very weird to go into a BBQ restaurant and have it be vinegar-based. I know there are some that do it (Scott's), but I never found it to be nearly predominant.
In any event, I just prefer the mustard-base. I'm South Carolinian through and and through and the mustard-base sauce is most identified with SC (yeah, yeah, I hear ya Georgia).