They get 3 vehicles slots in total as explained above:
Two, premium users can use two vehicle squads in a row, but after they are done with the second vehicle, they must play two infantry squads before using any vehicle, no matter how many vehicle squads the player has.
(tank-plane-two infantry - tank -plane)
(tank-infantry-plane-infantry-tank) is possible as well
I read all of your replies, and I withdraw my additional suggestions.
Let’s keep it as it is.
However, I want the non-premium users to have a way of getting a extra vehicle slot for flexibility.
If giving a slot to everyone is too much, let everyone be able to grind for more slots, just like Shiivex said.