There are 18 vehicles in total and all vehicles apart from 2 are unlocked through completing matches in which you receive an item which will come naturally as you go for the collect 200 items achievement. ( I had all of around the same time as I unlocked the 50 item achievement).
Use the same method as below for the Warthog (Hogsticker) to complete fast matches if you need to unlock these vehicles
To unlock the Armadillo you need to win the Season Championship
Easiest way to do this is Start a new season:
- 1v1 - Any team size is fine but with 1v1 you can basically score from the kick-off easily with 1v1
- Set it to 9 weeks
- 4 play-off teams
- rookie difficulty
You will then have to play through 9 matches + 2 more play-off matches to unlock the Armadillo.
To unlock the Warthog (Hogsticker) you will have to win a game with all other vehicles
Easiest way to do this is to start an exhibition match with the following settings:
- Any stage
- No bots
- Set Mutator Goal Limit to 1
Just drive forward and score a goal then repeat with all other vehicles and you will unlock the Hogsticker.