Vapor lock occurs when the vapor pressure of the fuel is higher than the surrounding environment. In boats, with "sucking" fuel pumps close to the engine, the pressure in the fuel line to the tank is reduced by the sucking action of the fuel pump drawing fuel form a fuel tank nearly ten feet away, and when heated, the gasoline may actually boil creating a vapor of gasoline which the fuel pump cannot handle, thus the name "vapor lock"; i.e. it occurs when the fuel pump, which is designed to pump liquid, loses suction as it tries to pump fuel vapour.

Simply put...fuel that is drawn in a vaccuum has a lower boiling point.

What can be done to combat this?

Shorten the hose.
Increase the diameter of the hose. <---inside.
Run the blower.
Install the hose in the lowest place in the engine compartment.
Do not use alcohol blended fuel.
Pressurize the fuel.

TMD