Karen, I don't think that lack of a vapor barrier is the problem. The reason is that a vapor barrier (properly called a vapor diffusion retarder) stops diffusion (diffusion is when water molecules pass though "solid" material) an diffusion is not the main mechancism for moisture transport into an attic. The contribution of diffusion is dwarfed by air exfiltration i.e. warm, moist air driven by stack pressure (the normal tendancy for warm air to rise), forced air heating system duct leaks, and now immensely strengthened by your attic fan, finds it's way from your living area up into the attic, or in your case roof cavity, where the moisture condenses on the bottom of the roof deck. Typicly there is a largish moisture source such as a wet crawlspace or basement or a furnace humidifier that is supplying the moisture. Even good ventilation where outside air enters at the eaves and exits at the ridge, can only carry off small amounts of moisture; cold air just can't hold much water. If your roof was rotted after 20 yr., you're probably pumping a good deal of moisture up there. Unfortunately, your attic fan, as I mentioned, will only draw warm air and moisture from the house at a faster rate; not what you want. The best solution to this condition is to seal the paths of air exfiltration. With a roof such as your's, the best solution is probably to dense pack the rafters with celulose. This will not only insulate but form an air barrier which will stop the moisture entering. You will also need to deal with the source of the moisture, since once it can't escape through the roof, it will cause the interior humidity to rise.
This concept may sound odd at first, but it works where venting fails miserably. You can (and should) do some further reading on the subject. Start at www.weatherization.com and then read the Air- Infiltration thread on TOH BB at www.pathfinder.com/TOH.