My daughter is having the same problem with her run - which considering she is a floor and vault gymnast uptraining to 4 piece (we are in the UK) is pretty serious - her vault run is the only thing stopping her moving up a level!
Her coaches looked at her vault run from all directions, and noticed that she is turning her knees out very slightly as she runs - she has taken ballet class since the age of 4 so it possibly stems from there. Anyway, the slight turnout is causing her to cross her feet over slightly behind herself - this is making her run inefficient - some of her power is diverted to a side-to-side motion rather than her power taking her forward efficiently. At the moment they are having her run more slowly, focusing on learning to bring her knees up, and run more on her toes, and really using her arms. They are having her do a handspring flatback. She is reporting that, even with the intentionally slower run, she is getting a better vault, pre and postflight 'feel bigger'. At the end of the gym session when they have a few minutes to go work on what they want (and most of the kids want to do somis into the pit) she is going back to the vault and working up this slow, deliberate run - it is not intuitive for her, but she's really trying. She is looking forward to gradually speeding up her 'new' run and trying it on a handspring vault
I don't know if this will help - I just know that sprinting as fast as you can is not enough if your technique is not right, you need to use the power that you have as efficiently as you can. I'm not a coach, just a mum of a girl who is struggling with the same problem!