The New Vaquero size is better for the .357 cartridge as the BH is way overkill.
That's one side of the coin, here's the other, the .44 frame size guns (Blackhawk, Vaquero) are hugely strong in .357 AND when you get that .44, you will already be used to the size and weight of the gun, from the "overlarge" .357 Blackhawk.
Fixed sights are nice for some things, but are only going to be "on" for one load, at one range. SO, either you have to shoot the load the gun is set for, OR you have to make a permanent change to the gun, it regulate it for the load you want. Then its set for that, and only that. OR you have to learn to aim "off" (Kentucky windage) for different loads, and leave the gun alone.
Your choice. I prefer adjustable sights. If you plan on shooting .38s most, odds are high they aren't going to print where a fixed sight .357 points. Sometimes, it is even the reverse.
here's an example, one SA I shot (not a Ruger), fixed sight .357, shot PERFECT with .38 special, dead on point of aim, accurate. .357 in the gun, same range, same everything except the ammo, every shot 2 FEET LOW and LEFT! Now, that much is unusual, and more than a bit extreme at regular short range, but it does happen sometimes. A handful of inches is more normal.
As to the change in Ruger's cylinder boring process, making the guns more accurate, I can't say from personal experience. Makes sense, though. Although I am happy with my guns made the old way. It is common for one chamber in the cylinder to shoot "away" from the rest. My first Blackhawk will put 5 in one ragged hole at 50 feet, with #6 about an inch away. However, at 200yards, #6 is off by about 6 FEET!
If your gun does something like that, and it bothers you, simply take a punch, make a small mark on the cylinder, marking the chamber that shoots "off". Then, when shooting for max accuracy, don't load that one.
