A vassal is a regent of a domain who has sworn to serve another regent. The regent who accepts a vassal is a liege. The oath that is sworn is called homage. This requires a Ceremony action. The liege has taken on the rights and responsibilities of some decision making for his vassal, and owes his vassal protection and justice. The vassal agrees not to act as an independent ruler, but to follow the principles and instructions of his liege in relations with other domains. A vassal continues to rule their own domain as they see fit. The liege takes on the obligations to protect his vassals from harm, since he has assumed their rights to fight other domains on their behalf. A vassal acknowledges the friends of his liege as his friends, and the enemies of his liege as his enemies. He is sworn to come to his liege's aid, militarily, in settling legal matters, diplomatically, financially, however his liege is in need. In exchange the liege provides protection and justice, connections, and participation in his broader realm that the vassal could not provide for himself as an independent regent.
A regent is limited to three domain actions per turn, plus some court actions. This limitation on time can be overcome through lieutenants and cohorts to some degree, but another option is a vassal. When a domain is large enough that a regent cannot collect all of the regency his domain produces, a blooded vassal makes a better choice than a lieutenant or a cohort, because in addition to extra actions, the regent will not lose as much regency due to the limits of their bloodline.

When a regent gives holdings or land to a vassal he can expect 20% of the revenues and regency to be donated back to the liege as an overlord tax. This applies to all the holdings and lands a vassal subsequently acquires on the basis of these resources. The only ways to acquire lands or holdings not bound by this overlord's tax is to inherit lands or to receive them as a gift. However, a vassal is not supposed to conduct diplomacy independent of his liege, so this act might be a violation of the vassalage agreement. A blooded cohort or lieutenant makes an excellent vassal.

When a regent has his own domain, but swears vassalage to a greater regent for the benefits, the liege expects no overlord tax, but can still expect his vassal to come to his aid, including financially as necessary. Perhaps more importantly, the liege directs the relations of the vassal with other domains.

When a war or other form of conflict is won, the victor may choose to accept vassalage from the defeated. A tributary domain is more like a created vassal, because he is required to pay tribute in the form of an overlord's tax, as well as potentially other burdens. As defeated foes, tributary vassals are generally not trusted as other vassals are, although like any relationship, trust can be earned.

Sometimes relations between vassals and their liege does not go as hoped. A recalcitrant vassal is one who breaks their oath. An unjust liege is one who fails to uphold their oath to defend their vassal.

A vassal who breaks their oath of vassalage, suffers a one time shift in the domain loyalty of all of the holdings held as part of their agreement.
A liege receives a +10 circumstantial bonus to Invest actions to recover provinces and holdings which were once granted to the recalcitrant vassal.

Just as a vassal can break his oath, so can a liege. Any time a liege intentionally fails to protect a vassal, he has broken his oath. At this time, a vassal can break his oath without the consequences identified above. If a vassal is attacked and the liege refuses aid, or if the liege declares war on his vassal, the liege has failed to defend his vassals.

A vassal is a kind of ally. However, allies are more or less equals. Both have equal claims on one another. If Avanil and Endier are allies, little Endier can pull Avanil into wars of its own making. When there is a significant disparity in the size of realms, large realms often prefer a vassalage agreement. For the smaller power, the conditions that would be imposed on a limited treaty of alliance would basically amount to a vassalage agreement without some of the benefits of being a vassal.