Hmm...strange. I would think you would be allowed to take as many classes (including online classes) during your "vacation" semester, as long as you were full-time previous fall and spring. Did you tell your adviser that you want to use the summer as your "vacation" semester? I don't know about grad school but for undergrads, I would tell the adviser that I intend to use the summer as my "vacation" semester (and enroll in the online class) and get a reduced course load permission for the last semester, if you need 3 classes for full-time.