When people are scared to try new vaults I have them try it onto things rather than over them. Say there is a retaining wall thats about 3-4 feet tall so rather than a drop on the other side it is level and you practice your form. Kong, speed, and lazy work well. And you arent going over anything you are just using the kong or speed vault form to push yourself as far onto this new leval. Here is a picture.
lllll /////////////////// more grass up here////////////
side view llllll
lllll Retaining wall of some sort
lllll
//////////grass //////////////////lllll
So walk to the wall and use the same technique these other people have been telling you but since you are up on the other grass there is no drop you just vault to a crouch up there. Usually there are obstacles like this even in rural areas or suburban neighborhoods, lots of people have a raised garden.
