Are nobles and vassals the same thing?
Likewise, is a vassal a noble?
A lord was in broad terms a noble who held land, a vassal was a person who was granted possession of the land by the lord, and a fief was what the land was known as. In exchange for the use of the fief and the protection of the lord, the vassal would provide some sort of service to the lord.
Keeping this in consideration, are vassals and knights the same thing?
Usually there is no difference because knights were vassals of the king. In exchange the knight would receive loot from battles, his own estate and position in the King's court. Every noble had to be a vassal to a lord. Even if you were a mercenary knight you became a vassal to the king you were fighting for.
Another important duty of a vassal was to attend to his feudal lord during court. He was also responsible for recruiting more men for his lord's army, protecting and managing his lord's manor, supervising all of the serfs and peasants who lived on the manor, and acting as a mercenary for his lord.