I agree that Mrs B is one of those individuals whose life is always in a crisis ("my poor nerves," etc) and her family has become quite used to her dramatics. In the film P&P2 it is obvious that Jane and Lizzy are somewhat amused by her outbursts when they occur within the family circle, and less so when they occur in public (Lizzy becomes very embarassed at Netherfield, for example). Within the family dynamic everyone has probably developed his or her own way of coping with these outbursts, including Mr B, who teases, or "vexes" her.

Teasing is an odd thing. There are many people, myself included, who feel that teasing is very often a sign of repressed hostility or anger felt by the one who teases. Teasing becomes a socially acceptable way of venting this anger and if the person who is the object of the teasing objects, the retort is usually, "Can't you take a little teasing?" I feel that Mr B's teasing, or vexing if you will, falls into this category. I think Mr B's disappointment in his marriage is palpable, and teasing/vexing Mrs B is the only acceptable way he can find to deal with an unhappy situation.