El Cid was a good vassal because he was a successful military commander and that is in essence what feudalism was, a system of military organization.  However, there are other characteristics which make El Cid a good vassal.  He is loyal to his Lord and he is courageous.  Furthermore, he is a devout and loyal Christian, which is all important to the nature of the chivalric code idealized by medieval society.  Furthermore, El Cid is generous both to his followers and to his Lord to whom he sends expensive gifts hoping to gain favor.  All of these characteristics make El Cid a good vassal by the standards of the medieval society in which he lived.