WAH, The New Vaquero is suited for the Colt loads. It's slightly larger than a Colt & Ruger suggest stay within SAAMI pressure ranges. You can still get well over 800fps with Colt loads using 255gr bullets & they will handle most needs with a six-gun. I too use the RCBS mould 45-270-SAA, it's a very good bullet & I think you'd be happy with it. Lee also makes a mould that throws a 255gr RNFP that has a metplate you can almost dance on. It's a great bullet also.
Good luck in you search for a load, The Speer #14 manual lists loads using the 45-270-SAA mould that are within the pressure ranges of your gun.
Frank