I understand the frustration, but I have my doubts about this solution. You could introduce a subject interaction to disallow vassals of obtaining access on their own, but it only takes a single participant in the entire war (your allies, your enemies, all of their subjects) to obtain access to grant the rest conditional access. In this case your vassal got it first, but I doubt that Tlemcen would have refused access to all of Morocco, Tunis, Tafilalt, Sus and Marrakesh, meaning that Tunis would likely have gotten access even if your vassal didn't. I suppose your proposed solution might have some narrow use cases, but as far as I can tell it wouldn't help you in the example you give, or in most cases I can think of.
Your suggestion did make me think of a related but separate issue that's bothered me from time to time (subjects
granting military access to others without my approval), but I'll file a separate thread for that