The 5.5" Ruger Old Army is simply that. The Vaquero is a cartridge revolver based on the Blackhawk frame and "Cowboyed up" for sport, competition and carry. I've never heard the 5.5" referred to as the Vaquero, unless that's a colloquialism from somewhere.

None of the Ruger Old Army's were nickelled. The 5.5" are stainless as well but were were given a polish as opposed to a brushed/satin finish.

The 5.5" is more rare than the 7.5". In the manufacturing period of ROA they were made for only a short time, relatively. I've owned both barrel lengths but only kept the 5.5". It does have a nicer "feel and balance" to it. I've found it every bit as accurate, but as it's a fixed sight option only, you either have to tailor your loads to where you want your POA/POI, or you hold off. I once read that the fixed sights were regulated for 35gns of 3F BP, an Ox-yoke wad and a .457 ball.

As said, under 700 would be the ballpark with condition being subjective.