there is an way to duplicate vehicles idk do you already know this but here it is
first you buy what ever vehicle you want to (statics dont work)
after you have bought the vehicle drive it close to your HQ and go persistent save the game
now go and place your vehicle to garage
restart the scenario
after restarting it will ask you "load persistent save" press yes
now you will see that you got car outside like you did when you persistent saved but when you open your garage you will see that the same vehicle is in there
now place your vehicle that was outside when you persistent saved to your garage so you can use it again (because its stuck on group after persistent save)
there is an way to duplicate vehicles idk do you already know this but here it is