I think it is more a convenience in writing. Having her live there allowed her to be on hand for all the various insanity that goes on, rather than "Oh, Vicky's having another one of her fits...better call Julia in town. She ought to be here in about an hour, so let's sit and wait." She was so hot for Barnabas, but I'm guessing she stayed at Collinwood rather than the Old House because 1) she liked indoor plumbing and electricity, or 2) the censors would not allow a single woman to shack up with a single vampire on the move.
It's entirely the same as the situations of the big, lavish nighttime soaps where fabulously rich family members who hate one another decide to live in the same house rather than buy their own house and get away from them. Having them all under one roof is not something you'd see in real life, but it's commonplace on soaps because you need excuses for them to interact. If they lived in separate homes, they could in theory never see one another and then you'd have no drama.