Tons of options for upgrades. Since you have the first step of identifying a budget, what items do you need from the new system; bluetooth audio, aux in, CD, handsfree phone setup?
Many of the posts on the forum offer makes and models of equipment that fit the 2009 Vibe. Of the $600 budget, I would factor about $150+ for sound deadening, especially the front doors and cabin area. if you are replacing the speakers. Sound deadening can make a $100 head unit sound like a $200+ unit just by reducing the amount of background noise your audio competes against.
I had put together details on removing and installing car parts during an audio upgrade This is for a 2005, the tear down for your 2009 is different but many of the concepts will carry through.viewtopic.php?f=11&t=40421
2005 #10,524 Neptune Vibe "Bandit" Auto, Moon & Tunes 207k
1991 Pontiac Bonneville 3.8L "Granny's Whip" 136k