On the one hand, lords and vassals inhabit completely different social strata and have different standards for behavior and comportment. On the other, as the example of El Cid himself indicates, there is plenty of room for talented, upwardly-mobile people to join the ranks of the nobility. El Cid is the son of a commoner, yet, by the end of the story, he is not only powerful, but considered noble.
The poem also allows modern readers the opportunity to glimpse what it means to be noble in medieval Spain. Certainly birth has much to do with it, but what really seems to define nobility is the willingness to give away the wealth that one has accumulated through battle. El Cid, even though he has been exiled, gives a percentage of the booty he takes from his enemies to the king and he gives money to the Church.
There is some truth to the idea that El Cid is simply buying nobility through his generosity, but in a medieval context, this was precisely the expectation. Nobility and leadership was defined as the ability to win battles and take booty, which could then be distributed in a way that cultivated influence. This, ultimately, was what a lord owed to his vassals. The defining characteristic of a lord in El Cid's society was military prowess, not really tempered by generosity, but in fact military prowess with generosity as its end.