A friend of mine died from what was probably vapor lock. Hot day, had been using Mogas mixed in with Avgas. There were a number of contributory factors. You'll be aware of the old one about not being able to brew a proper cuppa up a mountain. The water boils at a lower temperature due to the reduced pressure. It also plays havoc with timing your boiled egg. Conversely the pressure cap on your car radiator increases the pressure and so raises the temperature at which the coolant boils. Releasing the pressure on a hot engine is spectacular and dangerous as the entire cooling system boils at once.
Well vapor lock is just the same, fuel boiling. If you have a low wing and an engine driven pump then the pressure at the pump inlet will be below atmospheric and if it's low enough for the temperature and the fuel then vapor will form. If it does the pump will stop pumping because it's not designed to pump a compressible gas.
AAIB report here
Air Accidents Investigation: Fairchild 500172