I highly doubt an RX food is needed for weight loss and wouldn't buy into it. I mean, maybe if she's highly obese, but even so... I've seen dogs lose weight just fine on a regular food, and lowering the amount.
What is her ideal weight and how much does she eat (and what food)? Exercise?
Lessen the food, up the exercise.
Also, yearly shots (how old is your dog)? Personally, if a vet was still giving dogs yearly shots (that is, of dhlpp etc) then that vet is either making a lot of money off you or is completely uninformed. AAHA protocol calls for shots every 3 years but research has shown even that is too much.