That is what I suspected would be on your tank, that mult port valve with various functions although looks slightly different from what I have with that plugged off pipe fitting cap plug. The more steep angled tap I think is meant to shut in the whole valve to repair other valves but get more informed information then my guess on that. But if I am right about that, turning that valve unknowingly could shut off the flow to the vapor tap and fill tap "I think".
The valve you said had the hissing sound when you cracked it open, is it the one in the lower photo to the right and has the open threaded port ( and yes most likely its left hand thread ) as that looks to be the correct valve for connecting onto for vapor service. And like marshall said, you would require the proper pressure regulator right off of that valve before the line that you would run over to your garage. Depending on the tanks distance to the garage and if not an area you drive, you certainly could just lay down a temporary line for the winter but ultimately burying a line in the summer.
That small knurled knob with the small hole next to it, that is the liquid fill height indicator as a small tube runs down into the tank to the ... I think its 85% of total volume level, and when the propane is delivered that is how they tell when they have reached the maximum allowable fill level as liquid propane will spit out of that port, just like an NH3 tank being filled and using the same concept.
