If you don't have the money to get the Rodney Powell vest, just go for the Intec Ladies Flex Rider
I have this vest and I love it. I even went with a friend to a tack store that is juts getting into eventing and he tried one on and he loved it as well.
This vest is ASTM approved and certified, with the stamp of certification on the vest itself, and also - it does meet Beta Level 3 Standards of protection.
I love this vest, and I've used it for years now with no issues. Holding strong, comfortabl *in my opinion* and I forget it is on when I am riding.