That's actually fairly realistic, history is full of examples of people fighting each other even in the face of a larger outside threat.
True, but the very origin of vassalage is in the realisation that you won't survive without protection. In some places it really was 'protection' much of the same kind as when talking about the modern mafia.
Anyway, people should learn their lessons after losing a couple of wars to various minor powers that asking for the protection of someone big, let alone accepting an offer when it came, would be sensible.
The game shouldn't make it artificially hard for the player to gain vassals IMHO, in fact the opposite: the game should from time to time allow the player the experience of seeing someone ask to be a vassal (esp. de iure vassals, of course, and depending on opinion and other factors). That would look quite intelligent. OPMs being so stubborn seems to reduce the diplovassalage feature.