Braking is usually my first mod on any car...ideally I would just install rotors and calipers from a bigger car of the same generation...is such the case true for our Vibes?
I don't think there is anything OEM that you could upgrade to since you are looking at a GT model which already has the rear disc brakes, and it is the same setup as the XRS Matrix and Corolla. I think most people just get beter pads and maybe rotors if they want to improve the brakes. I don't think anything from a Camry or other Toyotas would work.
I did find this place, but don't know anything about them.