Okay sooooo if you read physical you can either grab this
or the three individual trades. Both 1946 & 1947 (especially 1947, AMAZING) deal with the vampires. 1948 kind of sidesteps around them for a volume, but features the lead character involved in the vampire stuff 1947, and the following book:
This one changes the name to vampire because it basically is the start of a proper spin-off into the vampire stuff (the historical BPRD stuff has jumped off into a separate thing, HB & the BPRD 1952 being the first of those). I would read all 4 if possible, but 1947 & Vampire are the best of the bunch. So far there is just the one volume of Vampire, but the overall plan is for there to be two more, although there is no formal time frame for that stuff to happen and be completed. The last word I heard is after the current Casanova mini wraps up, it's what the Ba & Moon Bros. will do next.