The first half being done right sets you up for a good 2nd half. Make sure your round-off is going straight over the verticle as you go onto your hands prior to snap down onto the board. Get fully extended to leave the board with open shoulders. Keep that extension into the table and push (block, and the faster the better) up into the 2nd half, versus "swinging" your feet into the salto. Be aware of which muscles to use to get into your salto shape, and "feel" those muscles working until your rotation is far enough around to "open" for the landing.
Vaulting, more than any other event, is a very rapid sequences of movements, each creating the "right" condition for the next. So figure out which movement is the first to "break down" and fix it. If you can video with a camera set at the starting end of the runway, you'll get any round-off errors. A good round-off viewed from the end should look like it is being done through an 18 inch wide passage way. Record the first half of vault practice from the runway, and then set up for a side view with the camera focused on the zone where you finish your hurdle to the first part of your salto. The more your body resembles a stick as you move from hurdle to round-off, to flight off of the board, with only a slight bend backwards as you come on and a slight hollow as you leave your hands, the better off you'll be.
That pretty well covers it.
