The Vaquero is one of, if not the most popular revolvers for SASS.
.357 Mag. Caliber is just fine for competition as long as you use .38's in lead at Cowboy velocities. There are several rifles made in .357, including Marlin, Winchester and Rossi (I believe marketed through Navy Arms) Several Ammunition makers offer .357 "Magnum" Cowboy loads which are loaded to low "Cowboy" velocities but use the longer magnum cases to facilitate reliable functioning in lever guns. These would also be appropriate, and your ammunition would be interchangeable.