My recommendations are to firstly get permission to make a mold of your armor from the manufacturer. If you post some pics of it, we can tell you with some certainty who you need to contact and how to do so.
My recomendation for you is to reinforce your original armor first, before modifying/altering it due to the potential for the weight of your mold to distort/warp the vacuum formed polystyrene during the rest of your process. Reinforce it, clean it up, make your changes, then make your molds.
As far as reinforcing your original vacuum formed armor, I suggest reinforcing it from the back with polyester resin and fiberglass. You can purchase these materials in most hardware stores. You will need some two part resin kits and a good supply of fiberglass cloth.
After reinforcing your armor you'll want to make a flexible rubber mold with a rigid shell (mother mold). There are many options for what type of rubber to use dependant on what kind of budget you have. After you have made the mold you would cast ther copies/modified versions in resin. Again there are plenty of options on what type of resin to use. Look in the forums for more info...
This is a very ambitious project altogether in you're new to this but if you take things one step at a time you should be able to get though it. I can send you more information as needed, just send me a PM with your questions.