The first version of the Vaquero will have only "RUGER VAQUERO" stamped on the left side of the frame. The second and current version will have "RUGER NEW VAQUERO" stamped on the Left side of the frame. The new Vaquero version has a mid size frame which is the same size as the original Blackhawk and more of the same size as a Colt SAA. The new hammers are slightly more Colt like in their appearance as well.
The new version is not as strong as the first version so you can't push the limits of cartridges like the 45 Long Colt if you hand load. Given this the original Vaqueros are a little more desirable to some and command a premium.