OK.....my tank is filled.....here's what happened....
I have a couple of vacuum pumps, so for fun I dragged one out, hooked up a vacuum gauge and connected the hose to the regulator output on the tank. The pump shown will easily pull above (below?) 28mmHg which is quite good.
One issue with this method is that when I remove the yellow plastic cap on the fill fitting air leaks back in the tank through the check valve....so I couldn't pull a vacuum at home then drive somewhere to have the tank filled.
I drove down to the general store in Trabuco Canyon.....they sell propane. I asked for a fill but mentioned that I would evacuate my tank with a vacuum pump first since the tank was new and filled with air. I had the operator connect the propane fill hose to the tank but not turn it on, then I plugged the vac pump into my van's 110V outlet (thank you inverter!) and pumped the tank down to around 26mmHg since the guy was waiting. Connecting the fill hose to the fill fitting eliminated the problem of air leaking past the fill check valve when evacuating.
Since my vacuum pump is not explosion proof, I had to be careful that the operator did not turn on the fill valve....this would have required aborting the procedure if there was propane vapor in the tank while the vac pump was on. Once the tank was pumped down, I closed the valve at the regulator and let the guy fill the tank.
He then opened the fill valve and the tank got a nice shot of propane, then continued to fill.
FWIW the tank is 5 gallon and the fill was 3.8 gallons.....cost me $12.90
The fill machine does an auto-stop according to the operator...not quite sure how as the vapor pressure shouldn't change much when filling.....