Another easyyyy achievement! Takes 2 minutes.
I had troubles finding the vehicles, so here is the solution. Found this explanation in the walkthrough: thanks to BAMMphil for that! He says:
Go to the "+" icon on the upper right - "Gameplay" - "Vehicles" - "Land" and place 5 Warthogs. (You can place any 5 vehicles you want, but that’s what I did, also left click the vehicle and press CTRL+D on your keyboard to duplicate it, to save you time).
(Halo 5: Forge (Win 10) Walkthrough
Note that the vehicles found under the category "props" (requisites) do NOT work for this achievement! Drove me crazy...