You must have DLC 2 for this.

Go on Americas Next Top Scientist and when you have defeated the helicopters, when it says take the truck to the news van, go to Stilwater University.

Go near the flats on the left side of the map and you will see a Vulture with the back doors open.

After that go in it and and press down twice. If it says mission failed, press B and you have the Vulture with the back doors open. You can store it at your garage.