When the player count drops below the required amount for the vehicle, you can't buy it anymore.
When one team has planes in the air, they have a significant advantage.
Solution: As soon as the player count is under the player count needed for the vehicle (from the server file), you can't rearm it anymore, but just sell it (tanks, helis on the repair pad, planes anywhere instead of service).
This way it will be more balanced.
(Also finally make a timer, so not many people can buy a plane at the same time and break the threshold)