When buying the 2GIG panel you need a 4G LTE module to connect to Alarm.com. You will also need to make sure the panel has the proper firmware to communicate with the panel as well. Which 2GIG panel are you looking into?
Do I need to remove the ADC cameras and sensors from Vivint before I activate with alarm.com even though I will not be using the Sky Panel?
Do you have Alarm.com compatible cameras connected now?
A list of compatible cameras can be found here. If so, they would need to be removed from the account prior to switching.
Sensors connected to the SkyPanel may or may not actually be compatible with the new 2GIG panel. Z-Wave devices would be.
RF sensors would not have to be removed from programming but Z-Wave devices would need to be removed from the parent network, either at the current panel or the new panel, before being added to the new panel.
Do the cameras need a fw update?
If the cameras are Alarm.com compatible, firmware is typically automatically detected and updated upon connecting them to the new Alarm.com account.