you dont need 1 instance per vhost, just 1 instance and you set your vhost using /bvhost <virtual.hostname> and then type /bjump for your vhost to be (re)set.
what is an 'instance'? Does that mean 2 users per psybnc? If so, then the answer is yes, the vhost on the 1st user will not change by adding a 2nd user with a different vhost.