It is possible to integrate additional payment processors, but to do so requires some heavy programming (as you have to interface between the vbulletin api and whatever payment processor api you are looking at).
Generally, if it was made for the vbulletin paid subscription system it would work with the pro version of vbcredits for selling credits. You can find a couple on vbulletin.org, but for others you will probably need to hire someone.
I personally would rather not integrate additional payment processors because there are so many of them, and demand for individual ones is usually quite low. That is why I opted to make it work against the paid subscription system, so you can leverage any existing processors, and if you were to have someone make more it would not require a third api to deal with.