You can't do it :?
I had thought about having a custom stocked grip made for my Super Redhawk, I looked into the law and a modern revolver falls into the SBR law, I would need to have someone install a 16" barrel on my SRH and THEN add the custom shoulder stock.
Numrich also sells board stocks for 1911's, and they do the whole CYA thing and say you must buy the 16" 1911 barrel and install the barrel first, then the stock, to be legal.
Certain C&R pistols are exempt, like Broomhandle Mausers, rare Lugers, some Hi-Powers, Webleys, Swiss 1882 revolvers, etc. that are made to accept a shoulder stock. They are C&R collectible guns and do not fall under the SBR law.
The other exemptions are the repro cap and ball revolvers like the Colt 1860 and the Dragoons, which can be fitted with those repro shoulder stocks. They are not FFL guns, but considered "non guns" because they do not use fixed ammunition and thus are exempt from the SBR law.
Bottom line, it's a lot of trouble, to have a custom made 16" barrel fitted to the Vaquero just to stock it.
If someone were to "just do it anyway" and the wrong person sees you with the gun at the range, you would be getting a visit from Agent Freindly......