Howdy
First off, the school marm in me will not let me answer your question without stating that both Single Action Army, and SAA are trademarks owned by Colt. Nobody else makes a Single Action Army or a SAA. If they attempted to produce a revolver with either of those names, Colt would sue the pants off of them.
Anyway, as has been stated, the replicas of the SAA as made by Uberti are very close replicas of the Colt. Although the parts are not necessarily interchangeable, the Uberti functions the same way as a Colt. The same size, which means the cylinder is basically the same size, which in turn means that hot loads cannot be fired though them. Any SAAMI spec ammo that does not exceed SAAMI Max pressure spec can be safely fired in an Uberti. This means most ammo you find on the shelf, as well as any 'Cowboy ammo'. Beware of the specialty ammo made by outfits such as Buffalo Bore. Some of their ammo is safe to fire in an Uberti, some is not. Read the specifications that Buffalo Bore has on their website to determine which of their loads can safely be fired through a Colt or a replica and which cannot.
Because the Uberti duplicates the mechanism of the Colt, they are not safe to carry fully loaded with six rounds. ALWAYS carry them only loaded with five rounds, and leave the hammer down on an empty chamber. This is not as big a deal as you might think, it is easy to get used to only loading five and lowering the hammer on an empty chamber.
The Ruger Blackhawk series is about 10% more massive than a Colt or a colt replica. This means there is more metal between the chambers, which in turn means they are able to safely digest the Buffalo Bore ammo that a Colt or Uberti cannot. Again, check the specifications on the Buffalo Bore website. Also, because the Blackhawk employs a transfer bar, they are completely safe to carry fully loaded with six rounds. A lot of folks like the idea of the adjustable rear sight of the Blackhawk, but frankly, once I sight an adjustable sighted revolver in, I seldom change the setting. I am used to Kentucky windage and elevation, which I also use with a Colt or colt replica.
Regarding the Vaquero, the only Vaquero available today is the New Vaquero. This is made on a frame about the same size as a Colt. Because of this, the amount of metal between chambers is roughly the same as a Colt or replica, and so maximum loads should again be kept to SAAMI Maximum pressure. There are those who will argue about this, but that is what I recommend. Like the Blackhawk, all Vaqueros have a transfer bar and are completely safe to carry fully loaded with six rounds.