The Vehicle Respawn Module has a bug when you hook it up to MQ4A GreyHawk or the other sides version of the GreyHawk.
The module works to respawn all other vehicles except for the greyhawk. At the bottom of the screen when you begin a preview of the mission a message appears stating:
"{BTS_fnc_moduleRespawnVehicle} Vehicle B Alpha 1-2:1 of type "B_UAV_02_F" has simulation "airplane", must be "carx", "tankx", "helicopter", "planex", "shipx" or "submarinex" ".
I believe what this is saying is that the greyhawk is labeled as "airplanex" however the module will only respawn it if its label is "planex".