In respone to how loud one vega 12 would get with 600rms, it all depends if you put the sub in a ported box or not. If you really want to feel the bass in your chest then port that thing and it will wang. If you need to save spave and want to hear more cleaness in the bass then go with a sealed box. In respone to if it will sound better than your old punch setup, tell me what amp, subs, and box you had going on in your old setup. Then I'll give my opinion if it will sound better.
I know you'll be happy with that vega though.