Counterproductive is putting it lightly. Point you daughter's coach at this forum and the general consensus from nearly every coach on here that it's not only a bad idea, it could be damaging her body.
Vault is my favorite event to coach and I can see no benefit in wearing weight while vaulting. I can see it destroying the ankles and growth plate. I dare say if a coach is resorting to using ankle weight to help vault, they have no idea how vault works. Hopefully this is a new coach and they are just confused.