That's probably the last thing you need to worry about. I've done a dash vault so many times, some crappy and some really awesome. Even on the crappy ones I've never felt like I'm gonna slam my back. The way I learned them is by straight jumping something. The straight jumping the same thing but a lower jump and then tapping my hands on the obstacle. Eventually work your way up to bigger things. And you won't hit your back if you don't go above your limits. Know your limits and you'll be fine.

For a dash vault u need to go fast

Meditate and imagine yourself doing the dash vault in both third and first person.

Try doing monkey vaghts and try to pike your legs forward

You won't mess up on this vault. All you need to focus on is getting over the obstacle, then planting your hands down to save that from happening.

Invishion yourself doing it then commit to it that's how a lot of parkour moves are

Don’t think

It doesnt hurt that bad as long as you arent vaulting bars or concrete

Try just jump over the obstacle before doing this vault

Try doing them on something that is soft