Nanu do you happen to know if this is the actual arms of R. Diáz de Vivar or arms that have been attributed to him at some later date?

Not taking into account the colors, that coat is very close to those of Catilla-Leon. The difference being the charge for Castilla is a triple turret (or towered) castle.

Blazon for Catilla-Leon (Ancient):

Quarterly first and fourth gules a castle triple towered or masonned sable pierced azure second and third argent a lion rampant purpure armed and langued gules crowned or