I have questions for any vaulters on here?

I sincerely think vaulting is one of the most underestimated equestrian disciplines. It's loads of fun. :)

Firstly...i think it doesn't matter your athletic ability when you start. If you can ride really well(which I'm guessing since you've been riding for so long), then I actually think that's more beneficial then being flexible and athletic. After all...you are on a horse, and looking at home on a horse's back makes the display a lot more appealing.

Nope, not at all. After they begin vaulting, they may take up some form of gymnastics or dance, just to acclimatise themselves to putting stress on their muscles and to speed up their progress outside vaulting lessons.

I'm going to make a very general statement and say yes, usually vaulters do other types of riding. I myself do vaulting in addition to eventing and dressage, and others on my team do other disciplines. As a matter of fact, they came to do vaulting through previous horse riding. :)

My advice...if you do start vaulting, it may be advisable to do yoga or pilates or something of the like to stretch your muscles and work on your balance, flexibility and co-ordination.

And of course...have fun! :D