The first thing that needs to be established is the difference between structureal ventilation and living space ventilation. Attic vents & crawlspace vents (I'll skip this point of discussion for now) are there to ventilate you structure to get rid of un-wanted heat and moisture from the areas of the structure that they are installed. They are not, nor should they be viewed as, there to manage your indoor air quality.
The fact that during the heating season you are having difficulty getting you indoor humidity down to a proper level (40% to 60% RH is good for health reasons. To prevent condensation on windows you will probably ned to lower the RH to 30% - 35%. I personally shoot for 35%RH in my house, but I still have some condensation on cold mornings.)tells me 2 things. 1/ You have a tight house. This is a good thing! Anyone who tells you that a house can be too tight is confused. On a properly built, tight house, too little humidity is never a problem. If a person needs to put in a humidifier to raise his RH, he apparently doesn't have a very tight house and would probable be better served putting his money into sealing it up. 2/ You have an insufficient air management system. On any house, but especially on a tight house, you need a system that allows for, and manages indoor air exchange. The most popular system to do this is an Air-to-air heat exchanger, although a central exhaust system with allowance for make-up air is a less expensive and effective solution. Either of these systems can be connecter to a humidity sensor (de-humidistat) to turn on when the indoor RH reaches the pre-set level. The reason this works is because, during wintertime, the cold outside air's "actual" humidity is low. exhausting out the humid inside air and bringing in the dryer outside air has the effect of lowering the indoor "Relative" humidity. Common de-humidifiers work best in cases of extreme humidity levels, and prove to be ineffectual in maintaining the lower humidities (There are some more serious whole house units that can handle it. These are vitually A/C units without the cooling.). Now, as far as your crawlspace. Since you already have the perimeter wall insulated and a vapor barrier on the ground, I suggest permanently sealing and insulation all crawlspace vents. Also, since you have you crawlspace inside you thermal envelope, insulating the floor is pretty useless, unless you have noisy mice and need a sound barrier.