I suggest that you use the users table that the forum software comes with for what ever integration you want to do. It's easier to do that, than to make the forum software use another custom users table.
As for the forum software, I don't think that counts. Anything works, from vanilla to Invision or VB. After all, you are not making modifications to its user system. You just query its tables :)
I have optimistic thoughts, even though sometimes (if not always) life's a bitch.