My DD can be like a wet noodle at times. She has a hard time keeping her legs straight, wobbles a bit. She is VERY flexible.
Sometimes it clicks and her legs are straight and she is tight, usually not. She would do so well in meets if she could get this fixed. That is why she is doing L3 again this fall. She has all her L4 skills and is working on L5, but is trying to "pretty" things up.
I have taken pictures of her doing BWO's showing her that her legs are bent. She will look at the picture (digital camera) and then go try again - each time she thinks her legs are straight, I show her they are not.
Wish I had some advice, but at least you are not alone
My DDs are also super flexible. But, they are those kinds of kids that have great form and flexible. ( I'm thinking they got their athletic side from their father, because they certainly wouldn't have gotten it from me) You could tell her to do a handstand on a wall, then tell her you are going to try to separate her legs and try not to let me separate them. That will force her to stay tight, and tell her to remember that position when she vaults and does bars. Or, you can put it this way- Say to her that she is almost having a competition with herself when she does bars. If she doesn't stay tight, she loses a point or something. I know that any athlete likes to win, even if it's against themselves. Good luck!
I'm assuming it's her back and that she is arching where she should be in a tight open position or hollow position. More core conditioning and handstand training will probably help. Make sure if she practices handstand she faces her stomach to the wall and walks up the wall holding in a hollow position, rather than kicking up back to the wall and pushing the stomach forward to rest the feet on the wall.
My dd also is super flexible in her back and has the same problem. Great advise about doing handstands facing the wall. My dd is supposed to practice handstands daily on coaches order to work on tightening her back but she does them back against the wall and has a sway back the whole time. One trick they do at her gym for keeping legs together is to hold a foam block in between her legs right above her ankles she has to squeeze her legs together to keep the block there. She has to do her whole bar routine this way.