How do I stop vapor lock?

The OP's fuel pump is also suspect. If the pump's check valves are good, then it should hold pressure against the carb's needle valve, for 20-30 minutes and prevent any boiling in between. If the fuel vaporized in the line, then the pressure in the line from pump to carb dropped for some reason: leaky needle and seat, fuel boiling out in the carb and thus opening the needle and seat, or leaky fuel pump check valves.