Sounds like you are underinsulated. The covered vent is only part of the problem. Sealing the leaking surfaces in the ceiling will help keep moisture out of the attic but doesn't fix the problem.
However, remember that people and other things such as humid basements and crawlspaces, hot showers, and fossil fuel combustion heating contributes a lot of moisture into a house. As we get better at sealing walls and using energy efficient windows, the moisture gets funneled upwards. So sealing up the ceiling isn't the end all of the problem as it may trap the moisture in the house below.
Condensation comes from the difference between the temperature and moisture levels of the air inside and outside.
You mainly need to control the humidity and moisture levels if they are higher than the house can dispell on its own without condensing somewhere or causing mold and mildew. Think of dehumidifiers and heat/energy recovery ventilators and opening windows to allow the humidity to equalize with the outside air.
Remember that mother nature will strike a balance. Any adjusting with one factor will cause a reaction somewhere else so there is no quick and easy and cheap answer.
Press control and f and start doing searches on this site on words like attic, insulation, humidity, moisture, vent, mold, mildew, and other similar words as we have discussed this at length on this site.
By the way, where do you live, Alaska, Hawaii, Frostbite Falls, Florida, Buffalo, LA, Denver, etc.? They all have different problems and ways of dealing with it.