IMO, fighter scores should represent the aerial combat results of a pilot.
The following ideas would contribute to a more precise scoring model.
1 - If you are killed by a ground vehicle, PT or ack, the death is added to your "attack" K/D ratio, not to the "fighter" K/D ratio.
2 - If you kill a grounded aircraft, the kill is counted as "ground object destroyed", not as an aerial kill (vulch problem almost solved).
3 - If you are killed while grounded, the death is added to your vehicle/boat K/D ratio. IMO, a grounded aircraft is more like a vehicle than a fighter. As an example, suppose a real P51 straffing 20 109s parked on the ground, should those 109s count as aerial victories for the P51 pilot?
This way, the fighter score will be related exclusively to aerial combat.