Knights were vassals. Dukes were (usually) vassals. So were barons and serfs. In an ideal feudal society, everybody but the sovereign was a vassal. The place that came closest to this ideal was England after the Norman Conquest. read more
The king was everyone’s liege (in theory). Some of his vassals were also lieges of their knights (fighting men) and squires (landowners). The squires were vassals of their lords, and through them, the king. Serfs were also vassals. read more