Place a slight (very slight) perk value on upping both?
Maybe have it so you can choose upto 3 vehicles for a formation, to represent a platoon. You can choose 3 tanks, 3 flaks, 2 tanks and a flack, or 2 flacks and a tank.
So say a 3perk cost an a formation of 3 M4(76)'s? And you wouldn't have to limit it to the same tank. You could up a panther, an M3, and a wirblewind.
Formations including perk tanks of value greater than the Panther have a 5% charge on top of the total of the price.
Firefly's have a set value of 3 perks added to its current price
Let you give 2 commands to the vehicles: 'Hold', ordering them to hold possition, and 'follow' which will have them follow you at a distance of 400yds. If you go more than 3k away without gathering your vehicles, they detonate like bomber formations.
This would require you to closely husband your vehicles unless you want to loose your perks.
M4(76) and T-34/85 cost an additional 2 on top of the current price.
Free tanks and flackers have a perk value of 1.
Supply carring vehicles are a flat value of 4 perks.