If your Vivint thermostat is not turning on AC, there are many quick fixes; the Vivint thermostat must automatically enable your AC when heating/cooling or setting an auto mode through the interface of the device.
However, there may be an issue that is preventing you from doing so, and we will review it in this post. If the Vivint is not turning the AC in the HVAC system, the problem may occur with the device's battery. The power supply to AC may have been cut, or the security switch has been cut on the furnace pipe.