The earliest time you can get this is during season 4 in career mode.
When the last few events BEFORE the season final of the WSR unlock you will also get another free car offered.
This will be your first Super Touring Car.
You can pick either the GC Automobile or the Volvo. Both of them have very similar stats, so it's only a question of taste which one you pick.
The other vehicle (which you didn't select) can be earned through a vehicle challenge (as usual in this game) on the Algarve (National) track.
If you win this vehicle challenge, you will also get the achievement.
So basically this achievement requires you to win a career event in a Super Touring Car and the vehicle challenge is an early and easy way to do so.
Thanks a lot to young tugboatz for posting the track name for the challenge.