If I understand you right, you are talking about the "kong" vault? If so, the easiest way (and in some ways, psychologically, the hardest way) is to force yourself to dive into the vault more.
In other words, if you force yourself to jump off a little sooner and a little farther, forcing yourself to dive into the object rather than running up to it, planting your hands, and attempting from there, the better the chance that you will have the forward momentum to get your hands farther on the object, and your legs through and land farther as well.
Sometimes people put a low chair in front of their object to force themselves to dive up and over it...I've also heard of using your bed as a good thing to practice konging over, since if you fail it's no harm done.