For me, it was two fears that held me back: 1) The fear that my upper body lacked the strength to hold/propel all my weight, and 2) The fear to commit to really jump/dive into the vault (largely related to fear #1, but also related to "clipping my feet").
I addressed the first fear by building upper body strength. I addressed the second fear by finding a large, soft, non-threatening surface that I could dive onto without necessarily vaulting over it, just to get the feel for the movement. For this, I used the full-sized bed in the guest room at my parents' house. Run at it a few steps. Jump. Dive. Propel with arms. Land on far side of bed.
I would try to force myself to take off further and further from the bed. I also focused on lifting my hips higher as soon as my arms came down. I'm not really sure how I do this, honestly, but I think a large part of it has to do with leaning forward instead of leaning back. In other words: to really follow through with my commitment to the vault, instead of trying to pull out it right at the moment of its execution.
This is still my biggest weakness with this vault. The difference between committing and not committing (at least, for me) completely changes how I use my arms once they have been placed on the object. If I'm not committed and end up leaning back and attempting to right myself too soon, then my arms become more or less a pivot point that I swing the rest of my body through, generally leaving most of my weight centered over the object from my shoulders down to my wrists. Think of my arms as the metal frames that hold the swingset in place, and the rest of my body as the swing.
When I do commit, my arms are used more to propel than to pivot, and I don't really begin to right myself until I'm cleared of the object. In this case, my center of gravity stays over my wrists, which means it shifts from my shoulders to my mid-section/waist as I push myself forward, leaning into the vault.
I hope that makes sense. Good luck with your training and keep us posted on your success rate!