Why is there water dripping from my ceiling AC vents?
Condensation is probably the result of uninsulated ducts running through uninsulated cavities in your home.  

The solution is to find out what section of your ducts this is occurring at and either swap the ducts with insulated ones or insulate that section of your home.

I recently learned this about my home.  I had uninsulated duct work in the uninsulated/not sealed crawlspace underneath my house.