I'd suggest finding something (Probably in a park area) that's a height that you can jump over easily every time, but not too easily. Basically just jump over it and try to bring your feet up so they clear the object at the same time. Then, gradually, try putting your hands on the object at the end, and eventually jump over the object with your body in the "L" shape, and do the dash. It's a fairly intimidating vault since it's really awkward, but it's really not that hard once you actually do it.