Recently I had a problem with heater deciding that while it was somewhere in the teens outside, it would not put out any warm air. After some checking I discovered that the thermostat was bad. I replaced the thermostat (and just added more coolant mixture versus flushing with the outside temp.) and once again had heat. That lasted about two weeks. I am not getting any heat under any conditions – defrost, EC, etc. although it does blow out plenty of cold air…
I have done some searching and found some leads – the mono valve and auxiliary water pump (the main water pump was replaced about 20k ago). My questions are:

1.Is there an easy way to check these to determine if they are the culprit?
2.What other possible causes are there?
3.I have always had problems getting heat from the floor vents. Doing some research it seems that could be attributed to a vacuum leak. I checked online for a mity vac and was overwhelmed by the different models available. Any suggestions on which would be preferable and where to get it? I would like one that is versatile (and reasonable) so that I can use it in a variety of situations in the future.

Thanks for any suggestions.