What is the purpose of the vapor valve? 

I have considerable experience with other LP engines, but have never run one which has a vapor valve.  Nothing I've been around will run worth a darn on vapor....can't get enough fuel to the engine to let you open the throttle without it leaning out.  

I drove to work every morning for several years on LP.  Never did anything but start the truck and drive out the driveway.  Only three times did I ever had an issue with freezing up, and two of those were because of low coolant.  The third was sub zero weather and it never quit running, just acted like I was running out of fuel for about a quarter mile.     LP wasn't perfect, but it sure beat the heck out of a quadrajet.