[baileyem]

Don
I don't think of myself as an 'old timer', but yes we used to experience vapor lock in the 'old days' (hey, I'm only 67). I drove a lot of Ford flatheads that would have problems in hot weather if the fuel lines and fuel pump got to hot. The cure was to either cool the fuel lines and pump with damp rags and cold water or to crack a fitting on the high end of the fuel line and release the air bubble. I never heard an explanation of what caused the problem. I just learned how to recognise it and how to deal with it. I think that we figured the gas got hot enough to boil, and that the boiling caused the bubble......from your explanation, I guess that is a reasonably close explanation.
I can see where an A4 stuffed into a hot, poorly ventilated engine compartment could easily develop 'vapor lock'.

Mike