I can confirm that you can't spawn in any vehicle (Spawn parameter set to true, mission fully loaded, debugging enabled).
The problem is that the player doesn't get the option to spawn any vehicle, so there's probably nothing wrong with the script,
just the initialization of the Vehicle Spawn sign regarding giving the players the option to spawn.
I've been looking for quite some time for another remake of the original insurgency in Arma 3 and I have to say... yours is the perfect answer for that.
Perhaps a few suggestions:
Extend the friendly area to the entire airfield and surrounding buildings, just to create a little bit of breathing room.
Patrolling units around the base in case the enemy does come looking?
Props around the base to increase the player's immersion (Perhaps a broken down hunter, mechanic etc. at the rearm area).
