A:
This is a great vest for a 9 m/o. As long as she's under 30lbs and the vest fits snug, it should be perfect. The vest shouldn't be to loose or overly tight.
I like the double straps between the legs, most infant vests only have one. I also like the zipper along with the chest straps for keeping the vest closed.
I bought this same one for my son last year and it was perfect on him. He was 5 mo in May and 9 mo in Sept when we put the boat away, and he wore it all summer.