I know Ai is hardcoded and there is nothing to do, but this is my experience:
Playing, with 1.2 i got a lot of protectorates, and i was never betrayed (and it was really funny to have a pair of faction being your protects), if you become protectorate, sometime the Ai mantain its protectorate with you, but playing vanilla BI (I play a lot this), i dont remember to have gained a Protectorate, and i have some absurd requests of becoming protectorate, and if i accept, in the next turn, AI betray me and attack, I also tryed some 1.5 mods like RTRPE and there is no loyalty, the AI always ask you to become protectorate, and if you accept they attack the same, i never obtained a protectorate in BI1.6 and in my tryout with rtw 1.5 mods. It seems that in 1.6/1.5 diplomacy is a complete Kaos for me.
Anyway i could be wrong, but these are my impressions so far.
cheers!