It's so hard on these cars to tell if the vibration is from the front or the rear brakes. "In the old days"...LOL...the park brake was part of the brake system. What I mean by that is you could slowly apply the park brake to determine if the vibration was in the rear. On these cars, the park brake is separate (a shoe setup inside the rear rotors) so you can no longer do that to determine which end of the car the vibration is coming from. The only way to know for sure is if it is severe enough to shake the steering wheel (this would be front) or to shake through the seats (most likely rear).
Stick to your guns! You're correct in saying it was not fixed in the first go around and they should still honour the warranty. If they had been smart, they would have told you no warranty to begin with, but now they have committed themselves. LOL
Good luck Sheena!
2010 2SS/RS - 6M Silver Ice Metallic
ARH LT Headers with MBRP 3" catback exhaust, Cold Air Inductions CAI, ZL1 inspired splitter, ZL1 Grille, Rear Fascia blackout, Gill decals, Aftermarket Hurst Short Throw shifter, Tune by Jannetty Racing.