Vacuum line cap is a short piece of rubber hose, like the air line tubing used in aquariums, or automobile vacumn lines, which is put over the nipple end as a rubber 'stocking' cap.
The hammer will not strike the cap on a properly timed and assembled bp revolver, the hammer striking the frame at the same instant the hammer face hits the cap, short of the nipple face. You should thus be able to dry fire a bp revolver without the hammer striking the nipple. However, in practice this is a very unusual situation; the hammer will almost always hit the nipple, thus the rubber cap for practice.
The problem is that nipples are finely specified as to thread diameter and pitch but not as to length, which is clearly a very important parameter. If the nipples were the right length, and the hammer properly set up, everything would work fine, but the nipple length isn't ever specified, so it's almost impossible to get things right.
Ruger Old Armys are a rather unique exception, however. Ruger's factory nipples are well specified and a factory assembled gun can be counted on to be in time, so Ruger feels they can allow dry firing. If the nipples have been changed to an aftermarket brand, all bets are off.
