How important is a vapor barrier in the ceiling to the attic?

Thanks

Very.
Moisture travels through wall and ceiling material very easily. The attic area is quite often effected by excessive moisture as mold often develops in this area from the condensation that occurs. Your ceiling is the area that is the most critical for insulation because of heat loss. Any moisture that goes into the insulation in the attic will lower its R value and cause you to spend more on heating and cooling as well.

The International Residential Code permits ceiling insulation without a vapor retarder provided that the attic above has sufficient ventilation: one square foot of net free ventilation area for each 150 square feet of attic floor area. If a vapor retarder is installed on the winter warm side of the ceiling assembly, then the net free ventilation area may be reduced to one square foot for each 300 square foot of attic floor area.

Determining the net free ventilation area of most existing vents is almost impossible, unless the manufacturer and model number is known, and then it may be possible to find the information on the manufacturer's web site. Generally, net free areas are in the range of 50% to 65% of gross vent area for vents such as screened louvers.

Although the code allows for this. I would never rely on such a thing.

I have seen hundreds of homes with more then enough ventilation Ridge, soffiit, gable and fans combined and the end result still is mold.

Once the vapor travels through the insulation during the winter or cooler months. It condenses long before it gets to the other side of the insulation. The result is damp, insulation ending up with mold, and lower R values. This is one area the code should be changed.

No argument there. I only quoted the code to illustrate that if one didn't have a vapor retarder, one had better have plenty of ventilation, much more than the average older home is likely to have.