You certainly could but I would wager the farm you'll end up with a destroyed mess of a good sixgun. Silly to even consider since the Vaquero is so widely available on the used market.
Rest assured, if you install a Bisley hammer on something that isn't, Ruger will replace it with whatever it came with if sent in for service. It's not a drop-in swap either, you have to grind off the rear of the hammer to fit the slot in the grip frame.
PS, "Old Model" is Ruger's official designation for those single actions produced before 1973 utilizing traditional half-cock lockwork. "New Model" is Ruger's official designation for everything produced thereafter. Since the Vaquero is a New Model single action with New Model lockwork, there is no such thing as an "Old Model" Vaquero. There is simply the Vaquero and the New Vaquero (that is, not the new model Vaquero, new Vaquero but "New Vaquero").
Vaquero is large framed.
New Vaquero is mid-framed.
York is in Great Britain.
New York is in the US.