I´m afraid it doesn´t work that, it only appears so because they pay you half their income (or something like that) which makes it hard especially for smaller nations to go on the offensive.
As for how historical it would be, I amnot sure. Rome allied with the Ptolemais already around 190 BC, and Rome gradually gained more and more power over them, but it´s hard to say when exactly it crossed the line between an alliance and a protectorate.