What role did vassals play in the structure of feudalism

Vassals were otherwise known as knights, as the two were in the same social standing. Vassals were also referred to as lords, and held a place between the two. A vassal held control of an estate and also fought under orders from the area's ruler. Whether this ruler be a neighboring lord or a faraway king, orders remained stagnant. A vassal also housed serfs (peasants) on his land, who worked the land in exchange for part of the crops and housing space, similar to the situation of African-American and white slaves in the early United States and during the Civil War in the South. 
Hope this helps, 
Ahawk