It looks like Robert Ludwig and I'm No Mechanic gave you some pretty good advice in your other thread. The advice to go ahead and replace the valve is probably not a bad idea. Even if it's not the reason for your no-heat, that valve is probably old and risky. (Another typical problem with those valves is just the opposite of your problem - they don't want to CLOSE completely so in the summer it seriously degrades the A/C performance.

One thing though about that valve - As I seem to recall (and I may be all wet here), but for some reason I'm thinking the valve works the opposite as you would think, meaning the vacuum pulls it SHUT, and limiting the vacuum allows it to OPEN. I can't say that with absolute certainty, but if you had it removed from the car you could suck on the nipple and verify how it works.