First off, Black Powder is no where near as difficult or as much of a pain as you think. Believe it or not, Black Powder is actually easier
to clean up after than Smokeless. Yes, it is messier, but with the proper water based solvents, BP fouling melts right off the metal with far less scrubbing than conventional Smokeless powder fouling does.
Anyway, no you cannot fit a Vaquero or Blackhawk cylinder into a Ruger Old Army. Just not an option. However there are two companies that make conversion cylinders for percussion revolvers, and they both make a cylinder that will fit into an Old Army, and they are both chambered for 45 Colt.
One company is R&D. Their products are marketed by Taylors.
The other cylinder is made by Kirst.
Of course, you will notice that these cylinders are not cheap. But they are very precisely manufactured, and work very well.
For what it's worth, I have a couple of R&D cylinders that fit into my percussion Remington 1858 New Model Armies. They are very well made and make it extra fun to shoot 45 Colt or 45 Schofield rounds in my Remmies.