There's not much to be done other than trying to destabilize their overlord.
your diplomatic options are quite limited for the moment, but there are some roundabout ways you can do. Like, you can pull Egypt into long wars to run up their war exaustion and then give them a lot of land to give them a lot of aggressive expansion, then turn around on them by supporting rebels.
assassinating ruler /hier may also have a destabilizing impact over time, but won't affect the vassals opinion or loyalty.
... all these things may as well not work. vassals hold on to their overlord-majors pretty tightly I find.