The reason for your problem is probably after the long drive the fuelpump have sucked fuel out of the tank but no air as entered to fill the space the gas had. After a certain volume the underpressure in the tank is so low that the fule pump can't pump any more. Remember that a pump can only suck maximum about 30 feet of a liquidpillar.
When you drive a short distance you will not produce the same amount of vacuum to reach the level where the pump can't pump anymore.
So the rule of thumb is "the volume that is pumped away must be replaced with air." That's the reason for the venthole in the cap.
So if I can correct you, the problem is not Vapor Lock.
Vapor Lock is the opposite where hot gas vaporize inside the carburators fuelbowls and create an overpressure that prevents fuel from entering the carb or in other cases overflow the bowl.
Heat in all forms is bad for gasoline (Except inside the cylinder). I had vapor lock problems and solved it by blocking the heatrunners under the carb. You know the channels that lets exhaustgas run under the carb.
Another thing is the bad quality of gasoline nowadays. When our musclecars were constructed the gasoline was of a much better quality and heatproblems was not that common.
We still have 98 octane gasoline over here but twenty years ago we had 100 that was more up to it and held for high compression.
Those were the days...
Jan
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