It's frustrating to come across a skill that you want to teach and a kid wants to learn and you just don't have the skill set to do it, but look at it as a huge opportunity to further your coaching abilities. In asking for help from the coaches who introduced this skill to your athlete, or other experienced coaches within a days drive or so, you can create a really great relationship to help both your athlete and your skills as a coach that will only serve to help your program down the road.
Explain the situation to the gymnast and parents and let them know you will explore potential opportunities at other gyms for her, or they can do that work themselves if they prefer, but that you cannot offer anything beyond working very basic drills and progressions for this skill (and by that I mean improving her fhs vault).
This isn't a kid asking to learn a skill that you can kind of make an "educated guess" at teaching and only risk teaching it incorrectly but no actual injury, this is a skill that poses a great risk if not taught with extreme care and it just isn't worth it.
USAG and individual states/regions offer some programs that might be helpful for you as well such as hands on coaching classes and clinics. Even if these particular clinics do not work specifically on that skill, you might be able to make connections and build relationships that will be beneficial. You can look here: Link Removed or on your state/region website for upcoming events or for contact info to see if anything is being offered in coming months (probably not with meet season about to be in full swing, but you never know).