Keep in mind that you can totally worship several gods at once in FR. When we choose a deity that just means that its our primary deity.
Vlaakith probably won't mind if someone also worships Tiamat, plus I doubt that you have to publically declare your faith for it to count anyway. The tricky part is explaining how your githyankee even knows about Ilmater or Lolth? It's not like they are common knowledge in the Astral Plane.
