spl = sound pressure level, pretty much BOOM!, eh don't really need to goto 15's, you can go 12's as long as they are put in a right box they can get pretty loud, and yes Vmax is good for the price
just means the 15's will play the lower notes a little better and you have more cone area for more air to move which helps make things louder but the sacrifice is the accuracy of the speaker the 12's will play best all around for mid and low bass
the JBL BP600.1 and the kicker 600.1 would work well with the VMAXs. if you want them to hit the hardest (or go the loudest) build or buy a ported enclosure, with 2.6 cuFt PER sub tuned to about 33hz. make sure theres enough space in your car for a 5+ cuft box
good recomendations as well steven has his in 6 cf total ported to like 32 hz and he hit like 142 db in his car but the box doesnt have to be that large it can also work well in 4 cf total its up to you concerning the space you have or want to give up
i didnt say they were cheaper, i said they would work well with the 2 vegas. the JBL can be had for around 200$ off ebay, and 240$ off other car audio sites. the kicker is a little bit more but can bring more performance out of the sub. if you dont want to spend that much look into hifonics or mtx