Only Client States/Feudatories, but not tributaries or Vassal tribal states.
Those last two are diplomatic dead ends. To move from tributary to client state you have to free the tributary. And probably go to war.
The issue is that when you can get a Client State/ Feudatory, you can also annex directly. At least in every case in my 6 hours playthrough. There might be exceptions.
I would have preferred annexation to be very limited and force you to go through the Client State route most of the time, which I feel is more authentic for most of the period.