I remember vapor lock very well. I've had my share of it. I once had a Jeep Wagoneer with a 401 that was terrible for it. When you shut the engine off, you could actually hear the gas boiling in the carb. I fixed it with an electric fuel pump mounted well away from the engine so it was pushing cold fuel from the tank all the time.
That thing was a powerhouse but it didn't get much better mileage than your truck. It was designed by a committee of idiots.
