Davidson's had Ruger to make the Vaquero in a dual cylinder 38-40 Win. and 40 S&W. They were available in stainless steel only and 4.625" barrel and 5.5" barrel lengths. I got one of each!
I also saw in the GunList after I got my Vaqueros someone was advertising a Ruger Vaquero in 40 S&W as a special run. I'm not sure which distributor had these made.
Hope this helps!
Slufoot