The back seats should fold down, but i dont recommend using that space for your subs, with the right box you should have plenty of room in the hatch. They have boxs specially built for camaros but i dont know if they make them for 2 15s or not. If not i would say get two 12 inch subs and a camaro box.
I have two camaros, do not face the subs towards the glass, you will get nothing but a lot of rattles and it wont be loud, you want a box that is made for camaros and firebirds. And there is room for 15s but i dont recommend it, two 12s will be plenty loud.
Vegas are great... My friend had two vega 12s in his camaro... Sounded great. No need for the 15s. QLogics sell custom boxes... Alright on quality if you dont have a big budget... Recommend Having an audio dealer build a custom box.