What do I use to cover an insulation wrapped duct? (Silver foil wrap?)

Duct Tape is good for a lot of things, except duct work! It will eventually dry up and pull away from the heat in the vent.

I would bend/dent one of the ends slightly so that it will fit easily inside of the other. Then I'd secure it with proper HVAC foil tape, followed by a clamp around it.

Foil Tape

_{(source: homedepot.ca)}

Metal Gear Clamp

_{(source: homedepot.ca)}

Owens Corning's 3" Type 75 SOFTR® Duct Wrap FRK, touts and installed R-value of 8.3.

If it's not in-stock at your local big box store, they should be able to order it for you.

^{_{I am in no way affiliated with Owens Corning, and do not recommend or endorse the use of any of their products or services.}}