Hi there! I am just a mom, not a coach... but my daughter just recently "figured out" the vault. Shes been doing the flat back for 3 years (2 years as prep-op and one as compulsory 4).
It is my understanding, after speaking with a Vault Judge (and as previous posters said), the girls are to run and accelerate as they approach the spring board, keeping their chest and chin up. They cannot studder step. The strides are to be even all the way through the run. When they punch the board, they should hit in the center of the board and stay in the flat back position through the landing.
We showed DD a coffee straw and that she should keep her body straight like the straw when she hits the board, hits the mat with her hands, and lands on her back. This visual aid seemed to help her as she went from an 8.9 average score to a 9.450.
Good luck to your gymnast! Vault is so subjective.