Yeah still not getting it...
You connect the yellow hose on a set of gauges to the vac pump. The high and low go where they go. You vacuum down the system and close the valves on the gauges. Sure, the yellow hose will pull some air, but that's why you burp it once connecting to the refrigerant, before opening the gauges.
Are you trying to valve it to keep the vacuum in the yellow hose by closing it off at the far end of the hose from the manifold? I suppose it could buy you a tiny bit of moist air protection, but not much.
The valves for refrigerant cans I've always heard leak out the refrigerant and don't seal well.