Wait, so you're saying that you're not pivoting and planting your feet, but rather, you always end up just vaulting? Sounds like an issue of committment. As you vault the rail, you will immediately spot where your feet will hit. At this same moment, the hand that you are pivoting around will be gripping the rail tight, and your arm will be mostly straight (because of momentum, that is, if you are doing this fast). You will basically swing around this pivot point of your arm and hand, like swinging a rope around your head with a weigth attached to it. Once you get a feeling for the centrifugal force, you will be able to control it in such a way that allows you to place your feet accurately.
How high of a drop are you trying this on? If you are having trouble committing to the turn, try it off of something without a drop and see if that makes it any easier. If it does, just drill the move as many times as possible until you feel 100% comfortable with it, then work your way up slowly. Personally, I would say that not being able to do a turn vault is a good standard for not trying to negotiate rails at any appreciable height (body height or higher).
Good luck!