| If the condensation can be wiped off, it is simply condensation, like what you would get on a cold mirror after a hot shower. It would not be surprising to see that, even with "energy efficient windows", if you have RH humidity in the house that is over, say 40%. The edge of an insulating glass unit is the coldest part, so that's where moisture will condense.
There are many reasons it occurs on the bottom of the glass only. Most of it has to do with airflow. Best thing to do is to try to reduce your humidity somehow. You might also be able to reverse the direction your fan blades turn. in the winter, it's best to set the fans to blow upward rather than downward, which draws colder air upward, mixing it with the warm air near the ceiling.
One of the reasons vinyl is touted as being "better" than wood is that a little condensation like this won't harm the finish on the vinyl, like it would harm the finish on wood. | 
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