That has got to be the biggest load of horse hockey I've ever seen in my life. The fuel pump HAS to be able to lift fuel out of the tank even if the pump impellers are spinning in air (or vapor). If it couldn't do this, the pump could not prime itself after you changed the fuel filters, or if you happened to run out of fuel, or whatever. Given that the pump is self-priming and the fuel system is recirculating up to the upper fuel bowl, a "vapor lock" is highly improbable, if not just down right impossible.