Ignition timing won't cause insufficient vacuum to run the system- as long as the engine's running there's enough vacuum to keep the fuel system "turned on".

As far as the vacuum hose routing- the vacuum shut off ( the first big round thing in line from the propane tank) should have a small vacuum line from the mixer (carb) feeding it.

Is there frost developing on any part of the propane system- this is an indicator of a blockage w/the subsequent pressure drop.