what are the conditions needed for vapor lock and what is it?

Vapor lock is a condition where the fuel actually gets hot enough to change to a vapor before entering the carburetor or throttle body. So if the temperatures under the hood are extreme, its a good chance this will happen. It causes a loss of fuel pressure.

In hot weather and driving a warmed up engine with the fuel pump mounted on the engine, sometimes the fuel can get hot enough in the fuel line from the tank that it will boil , or vaporize before it reaches the pump. The fuel pump cannot pump vapor, therefore, no fuel reaches the cylinders, and the engine will stop.

vapor lock occurs when water gets into a combustion chamber. water, being no compressable by nature, makes the pistons stop their travel prematurely, putting strain on the pistons and crank, eventually causing enough damage to make the motor seize.