you wouldnt really want to give one 12" VMAX 500watts. they only need 300 to reach full excursion, im afraid that the extra 200 might cause distortion. 2 10's would be a better match for that amp being that they only need 200watts a piece for full excursion and would only be overpowered by 50watts each. the SPL should be pretty close on either setup but the 2 10s would be more efficiant. you might want to consider buying a higher grade sub that could handle the 500watts your amp puts out. consider the 15" VMAX?
Trying to put something in my wife's car.... another option would be 250W to a single ported 12" vmax (or other 12). That should be enough power to go ported ya think? I may have to do something like a CompVR or An Orion P12. They can handle more power and are both cheap here locally.