In theory, vellón is an alloy of copper and silver, as the first DLE definition cited by Bea indicates:
"Aleación de plata y cobre con que se labró moneda antiguamente"

In English it is called "billon", derived from French:

"billon. An alloy formerly used for coinage, containing gold or silver with a predominating amount of copper or other base metal."
https://en.oxforddictionaries.com/definition/billon

Up to the end of the sixteenth century, Spanish vellón coinage did indeed contain some silver along with copper, though as silver became more scarce it was reduced to a very small amount; by 1591, it was as little as 0.35%:
https://books.google.es/books?id=WsSlZqEvTZ0C&pg=PA143&lpg=P...

However, before the end of Philip II's reign, the minting of vellón coinage consisting of pure copper with no silver at all was already contemplated. An order was issued on 31 December 1596 to mint coins in copper "sin que lleve ni se le eche ninguna liga de plata", and although this was rescinded a month later, soon afterwards coins which continued to be called vellón but were actually pure copper, with no silver at all, began to be minted in huge quantities; Philip III's Real Cédula of 3 June 1602 ordered that "la moneda de vellón que en adelante se labrase fuera sin mezcla ni liga de plata y de la mitad de la agora corre". From then on, apart from a period in the later seventeenth century when they went back to minting in copper-silver alloy, vellón was in practice pure copper, although the old name was retained.
"Al principio, por no haber demasiada moneda fraccionaria, el vellón se encajó bien. Pero ya no era vellón, sino cobre"
"La modesta moneda, primero, de vellón, después, de cobre sólo (aunque comunmente llamada vellón) [...]"
For full details, see pp. 50-61 here:
Manuel Vilaplana Persiva, Historia del real de a ocho
https://books.google.es/books?id=SGNJPtOQvgQC&pg=PA50#v=onep...

By the time the first Spanish Royal Academy appeared in 1726-39, vellón was defined simply as "la moneda de cobre Provincial de Castilla", and so it remained in later editions; the definition was "moneda usual de cobre", and the meaning of vellón as an alloy for coinage was not even included until 1899, and then with the definition "liga de plata y cobre, con que se labró moneda antiguamente" (my emphasis): i.e., a long time ago, not recently.

It is not surprising, then, that in nineteenth-century English sources "reales de vellón" were called "copper reals": that is what they actually were:

"At Bilboa, Madrid and Malaga, accounts are kept in reals vellon, or copper reals of 34 maravedis."
The Shipmaster's Assistant and Commercial Digest (1837)
https://books.google.es/books?id=QzhkAAAAcAAJ&pg=PA392&lpg=P...

In the light of all this, although I myself have translated "reales de vellón" as "billon reals" in the past, I think it would be better to call them "copper reals". The fact that they continued to be called vellón officially might justify using "billon", but since they were actually pure copper (after the sixteenth century) we might as well say so. "Billon" is a very unfamiliar word in English, familiar only to specialists and unsuitable in a tourism text. Readers would be quite likely to think it might be a typo for "billion". "Copper" is much more easily understood, and also accurate.

I would in any case prefer to retain the Spanish term (which would go in italics in English), and then add "copper reals" in parentheses. As for "reals", you can either use this form, with an English plural, or retain "reales" in the Spanish form (preferably in italics). You find both.