Vapor lock happens when a gasoline engine seizes up because the overheated engine has boiled the fuel, making the pistons stick in their cylinders.
Vapor lock occurs more often at high altitudes, because gasoline boils, or vaporizes, at a lower temperature as you get higher up.
If your car gets vapor locked, carefully loosen the gas cap, open the choke flutter valve in the carburetor, and wrap a wet rag around the metal fuel line leading to the carburetor. Then let the car cool down for at least half an hour.
Vapor lock usually doesn't happen with newer cars. Older cars have engines with sucking fuel pumps, while newer engines use a fuel pump right next to the engine that pushes the gasoline under pressure into the fuel injection system or the carburetor. | 
