I really don't know the answer - but my gut feeling is that if it is no longer the home of anyone who has welcomed the vampire in, they would find the way barred again. So if B had been happy for the vampire to be in the house when A lived there, chatted, gave him/her tea and biscuits, then the welcome would be assumed to be from B too.

If C was never friendly, left the room when the vampire arrived etc. then when B left too I think the vampire might find the way barred.

Fascinating question with a whole lot of possible answers - I reckon that you can more or less take it as you like.