Vehicles are used to export eggs. There are eleven tiers of vehicles.
The base price of buying or upgrading a vehicle depends on how many of that level of vehicle you have immediately before your purchase, not on how many vehicles you have of any other level.
You can look in your vehicle tab by tapping the import station. If the tab is red, you will need to upgrade your vehicles soon. If it is orange, it means that it is almost full. If it is green, it means that there is a lot of space to go.
Certain types of research will increase the carrying capacity of vehicles or decrease the price.
By default, there are four vehicle slots, but upgrades will allow you to have up to 14 vehicles. (the 14th is extremely hard to obtain due to end-game glitches)
Note: New Table Format. Shows minimum and maximum prices. Does not include epic upgrade discounts.
Trivia - Tapping on a vehicle makes it honk.