Howdy
Whether or not it is 'right' the unfortunate fact is that a lot of Vaqueros and Blackhawks leave the factory with this problem. And it is not limited to Rugers either; I have two 2nd Generation Colt Single Action Armys that I had to do some fitting to get the latch to properly retain the cylinder base pin so it would not move. The difference is, with a Colt the pin will keep working its way out until it falls on the ground and gets lost in the grass. With a Ruger, once the pin works its way out about 1/4" or so the transfer bar tends to jam under the firing pin locking up the gun. If you can't cock the hammer properly it is always advisable to check if the pin has jumped forward. If it has, the little spring plunger at its rear is no longer doing its job of pushing the transfer bar to the rear as it rises. Without that rearward thrust the T bar can easily jam under the firing pin as it rises. So a Ruger will usually let you know when the pin has jumped forward, a Colt will usually not. The worst case with a Colt is that if the pin jumps forward enough that its rear end is no longer in the frame, it can easily lose its alignment with the frame, allowing the cylinder to sit at a slant, which can cause big problems.
I don't fault Ruger too much for this problem, I have no idea what their tolerance is for the true position of the cylinder pin hole in relation to the latch hole. The diameter of the latch will also come into play as well as the depth of the locking slot. Given that these guns are mass produced, I'm not surprised that some slip through with poorly fitted locking latches.
Although a stronger spring can help, the real solution is to make sure the engagement between the latch and the cylinder pin is positive enough to do its job. If the latch is only extending partway, relying on a spring to keep the pin in place is only treating the symptoms, not the cause.
As for how far the latch should extend, compare how far it snaps in with the cylinder and pin in place to how far it snaps in with no cylinder or pin in place. With the cylinder and pin in place, the latch should snap in just as far as when it is not engaging anything. If it only goes in partway with the cylinder and pin in place, there is probably interference between the parts, not allowing the latch to completely engage the slot in the pin.
Over the years there have actually been a couple of different designs for the slot on the pin that engages the latch. The old fashioned style was that the pin did not have an alignment notch on it. The pin could be inserted in any orientation, because the locking slot was a groove that ran completely around the pin. Later, Ruger changed the design slightly, including an alignment slot on the pin, so it could only be inserted in the frame oriented one way. With this design, the engagement slot on the pin was cut horizontally across the pin, it did not run all the way around the pin. More recently Ruger has gone back to the design with the slot running all the way around and no orientation slot. The 357 Mag New Vaquero I have in my hand has this type of pin. Unfortunately, the other design was superior. With the slot running completely around the pin, there is less surface engagement with the locking latch. With a slot running across the pin, there is more engagement surface. So it is easier for the pin to 'jump' the latch with the circular design. That is why the Belt Mountain pin has the orienting feature on the pin, and the slot cut across the pin. I have that directly from Kelye at Belt Mountain, he feels it is a superior design. It is probably also more costly. The other design can be completely cut on a lathe or a turning center, all in one step. The design with the horizontal slot requires a secondary operation, off of the turning center, on a milling machine to cut the horizontal slot. More operations means more cost. I would bet that is why Ruger went back to the inferior design.
Anyhoo. I have fit Ruger pins and Belt Mountain pins to Rugers and Colts. I have not messed with springs or reshaping any parts in the latch, I did all the work to the slot in the pin. The trick is to get the latch to snap in as far as possible when engaging the slot in the pin. That way you get the most engagement between the two surfaces. All that is required is a tiny jeweler's rat tailed file and a black marker like a Sharpie. This trick is much more successful with the Belt Mountain style pin with its horizontal slot. Pull the pin out and black the slot completely with the Sharpie. Then put it in place and snap the latch a few times. Pull it out again and examine the ink. The ink will scuffed away where the latch engaged the pin. Take your tiny jeweler's rat tail file and remove just a little bit of metal and repeat the process. Did the pin snap in farther after you removed a little bit of metal? If so, the new scuff mark will have moved slightly. Remove a tiny bit more metal and check again for engagement. Repeat this process until the latch snaps all the way home when the cylinder and pin are in place. Be careful to only remove metal at the scuff mark, no place else. Like I said, this process is much simpler with the horizontal slot type of pin. With the pin in my New Vaquero, that runs around the slot, you have to remove metal all the way around the pin to ensure that no matter how you insert the pin there is always plenty of engagement. You could try chucking the pin in a drill press and carefully removing metal with your file, but be very careful, it will be very easy to remove too much metal. You don't want the pin to be too sloppy.
Regarding how much slop is allowable, there is a tiny bit of forward and back slop on some of my pins, others have no slop at all. So a tiny bit will probably not hurt anything.
Regarding the Belt Mountain pin with the locking set screw, I do not recommend it. I installed one on a Vaquero and quickly found that just a tiny bit too much tension on the set screw would bend the pin a tiny amount, just enough to cause the cylinder to bind. Better to properly fit the pin to the latch than to rely on the set screw. Besides, it is a real pain to keep that tiny allen wrench handy at the range if you want to remove the cylinder for any reason. I do wholeheartedly recommend the non-set screw type of Belt Mountain pins. The Belt Mountain pins are more precisely controlled for diameter and straightness than the Ruger pins, they are centerless ground, they are not cut.
By the way, originally the Colt SAA employed a screw to retain the pin. The screw entered the frame at an upward angle of about 45 degrees and engaged a slot in the pin. The spring loaded latch was not introduced until around 1892, if I recall correctly. The screw type of retention was much more positive than the spring loaded latch, even back then. But shooters preferred the convenience of the spring loaded latch over a design that required the shooter to keep a tiny screwdriver handy. But the spring loaded latch is not very reliable if it is not properly fitted.
You could use a hose clamp. Just kidding 
