@timesen
I always supposed that the agreement between lords is that the vassal would help fight his liege's wars in exchange for being given fiefs. Lords come with soldiers and connections, and sometimes even their own land, which are very much more valuable than having them pay tax. Sometimes, a vassal can be just as powerful as the king himself, and imposing tax on them is impossible. Even then, a disgruntled lord can be more a threat to the realm than a foreign invader is. If all the king wants is to fill his coffers, he could just keep the fiefs to himself! Managing lands? Leave that to the lieutenants, ministers, and magistrates! Lords can employ such people without giving them any power.
But I suppose this is the type of kingdom where the king has enough power to impose taxes like this. Historical kingdoms are no stranger to enforcing some arbitrary taxes. As long as the lords would and could agree to it, the king can tax them any which way he wants.