Here's a few pictures from an earlier thread of an actual gasoline vapor bubble in the sediment bowl where it could be watched. With the engine idling, I could watch the bubble grow until it completely covered the bottom of the screen and would then burp on down the line to the carb and start all over, over and over. I'm pretty sure the bubble is comprised of the lightest fraction of the gasoline that vaporizes as soon as the relatively-cool gasoline arrives in the warm engine compartment. Amazingly, the motor barely hiccuped when the bubble passed.
First, here's a picture in which you can see the shiny vapor bubble just beginning to form where the gasoline enters the bowl. Note that this is a gravity flow system with the motor running, and vibrating, and the bubble is NOT going back up hill into the tank:
The rest of the pictures are on another day with the screen installed. First, here we are just before the bubble has started to grow:
Here's a picture with the bubble grown to about half its ultimate size. Because everything is not perfectly level, the bubble lolls off to one side:
Here's another shot with the growing bubble covering most of the screen:
Here the bubble has completely covered the screen (sorry, not so clear photo). Shortly after this, the bubble suddenly blurps out the bowl outlet, down the line, to the carburetor:
All the while during these photos, the motor ran normally. However, it's not hard for me to believe that under more severe conditions, similar vaporization could occur in the fuel line and the carb, eventually overwhelming the poor motor's ability to cope.
Several years ago I experimented with a clear plastic in-line fuel filter that I spliced into the middle of the fuel line: I could watch with my own eyes as the filter filled up with vapor and stalled the engine--true vapor lock, in a gravity flow system being vibrated by a running motor! Sorry, that was before the days of digital camera and cheap photos. Also, I believe that the volumes involved establish that we are dealing with gasoline vapor here, not air.
Steve