i've always declined offers at protectorates...if fact i wanna attack them the next turn if i can just for trying...if there was a squishy button u could use on the diplomats i'd use it often...
i recall offering it once though to scythians when i was playing briton or german...they accepted, never attacked me but there was only one border...
i only ever did that once though because it seemed to make me poor quickly to have a protectorate.