What oath did a vassal swear to his lord?
Also asked, what did a vassal promise to his Lord?
As part of the feudal agreement, the lord promised to protect the vassal and provided the vassal with a plot of land. This land could be passed on to the vassal's heirs, giving the vassal tenure over the land. The entire agreement was called a fief, and a lord's collection of fiefs was called a fiefdom.
Also to know is, why did vassals have to swear an oath to remain faithful to the King?
It included the swearing for remaining faithful to the lord, the act of homage preceded it. The Vassals oath was a promise not to harm his lord or damage his property, it was a recognition of the assistance owed by the tenant to his lord and the protection owed by the lord to the tenant.
Fealty was an oath of fidelity made by the vassal. In it he promised not to harm his lord or to do damage to his property. Although homage had to be rendered directly to the lord, fealty could be given to a bailiff or steward.