Vassals also face the problem of sending their main source of income to inevitable death in a war. Conscription probably happens now and then, when the need is dire, but it's not a sustainable method of waging war. Many lords could conceivably hire out their excess soldiers as mercenaries to lords closer to the border, who can put them to lucrative use against monsters and warlords. Yet another reason why the king had such a hard time getting his armies together in time to repel the invaders.