I'm sure it's the original/large frame Vaquero. I believe the only "discreet" .40 S&Ws were the special/limited San Diego guns. Then there were the .38-40/.40 S&W convertible Vaqueros--also a limited run. The only ones I've ever seen were 5.5" stainless. Anything with a 10mm I think would have been one of the "Buckeye" special .38-40 Blackhawk convertibles--correct me there--but of course the 10mm cylinder could very likely be fit to a "40" Vaquero as well. Same frame so don't know why not, though you'd want to have a pro smith fit and time it.
I think the .38-40/40/10 convertible is a no brainer for the midframe FT/New Vaquero, though some have speculated whether there's enough space for the .38-40 (and .44-40) rims--as the overall case (spec) is wider than .44 Specials or .45 Colts--for Ruger's comfort level, from the perspective of a manufacturer's standards vs the dimensional differences with ammo out there (but custom smiths I believe have done it, who can build to a different target/objective) If not, a special run of strictly 10mm/.40 would be nice also.