Nearly a textbook answer SS. Vapour locking can cause a large vapour bubble in the fuel line which maintains a small amount of fuel in the carb bowl / injector, normal fuel pressure - but means that you've got stored up a period when suddenly fuel stops flowing (usually just as you have left the ground!).
A return line means a continuous trickle of cold fuel through the lines, and also an escape route for vapour bubbles. Thus, there's a much reduced risk of vapour locking - which otherwise can cause an EFATO (with no subsequent fault found usually) after the aircraft has been sat in the sun for a while (sun! - remember that?).
G