Sorry, it is still not clear to me if you checked the line going to the solenoid (not distributor).*
Assuming you did, I'd say it always has vacuum as well. That be the case, the thermal control valve must be bad because, regardless of temp/microswitch, it is impossible to get a yes-yes condition no matter how you route the 3 remaining lines if the thermal control valve is functioning properly.
* It is important because the solenoid can be connected backwards causing it to not vent properly and hold vacuum on the distributor advance line after the solenoid is deactivated...this can fool you into thinking vacuum on the distributor line is there from the vacuum control valve when it is residual/the advance diaphragm supplying it... Not to mention 6 possible ways to route the lines can make ones brain hurt and a 4th line would make it 10 ;-)
lol-I'm guessing I wasn't clear... The point there was that, when the engine coolant is below 104F, it will perform poorly and be prone to backfire (too lean) with the vacuum to the WUR blocked off. A bit risky, imho.