insulating duct work in attic

First and foremost, seal all the seams with mastic or a proper tape.

Depending on the type and how much insulation you have in the attic, many times you can run it across the attic floor and blow loose insulation over top to a depth that will insulated the duct work at the same time.

They make sleeves. Imperative to get the ductwork completely sealed prior to application.

You can actually take batts and wrap them around the ductwork as well for more R-Value but the pre-made sleeves are more user friendly for the initial insulation and vapor barrier.

http://www.handymanhowto.com/insulate-round-sheet-metal-air-duct/

No. Do not mess with the venting of the attic.