Will a cylinder from a Ruger Blackhawk convertable fit and work in a Vaquero?
Specifically, a .45ACP cylinder in the .45Colt Vaquero?

Maybe yes. I'm guesing by "Vaquero" you're referring to the Vaquero and not the New Vaquero (Too many folks don't get the differetation and confuse them) Theoretically the cylindr will fit and function but the importnat thing is that it's timing is correct. Many will simly drop in but it's best to double check the alignment and timing BEFORE shooting it.
There actaully was .45 Colt/ 45 ACP Vaqueros made in small numbers as a distributor model I believe for Davidson's.

Ruger has and still does make some Black Hawk's with interchangable cylinder's. However they are FACTORY MADE.

Ruger's are NOT MADE for their cylinder's to be interchangable form one revolver to the next. IF you do this you are asking for real trouble.

Ignore anyone who tells you differently!!!

If you want an additional cylider the revolver must be sent back to the factory. I am not talking about the BP revolver's. Tom.

Notice I stressed having the timing checked! They can and have been interchanegd safely in some but others HAVE needed timing correction or simply didn't work. Ruger will NOT fit a cylinder to a firearm in which the final product is of a configuration that wsn't a cataloged model.