Final answer:
Vapor lock is caused by the increase in vapor pressure when a liquid is heated, and it is of concern because it can lead to explosions in sealed vessels.
Step-by-step explanation:
The primary cause of vapor lock is when a liquid is heated, its molecules gain enough kinetic energy to escape from the liquid and enter the gaseous phase. This creates a population of molecules in the vapor phase above the liquid, resulting in an increase in pressure known as vapor pressure. Vapor lock is of concern because if the vapor pressure becomes too high in a sealed vessel, such as a car's gas tank, it can lead to an explosion.