How can I move a wall vent about 2 feet from where it is now. Same wall.
Also how do I cap one off

A few picts would help a bunch in getting the info you want.
What size vent?
What is this vent for?heating?
What do you have for duct work?

Offsets/flexduct/many ways to do what you want.

Capping off a vent is easy if you know what you have to cap.
Size and type of line you want to cap.
Get back and someone can help.

Well you will have to cap it off and move the duct to another joist. If it is round duct you can get a cap at Lowe's, if it is square you will have to fab one up out of metal