How do I close and seal an old external vent?
I would probably use some rigid foam insulation (2 or 3" is probably good, it's generally rated at R7 per inch - and you can stack it, it doesn't have to be a solid piece), and then use spray foam to seal it up around the edges and hold it in place. 

It looks like a metal sill all the way around, so that's going to cause a lot of heat transfer anyways - ideally, to make it as warm as possible, get rid of the metal. You can also then pull off the trim and just put some backing in, and a piece of drywall (or plaster) to finish up the inside.

The most important part is making sure it's sealed to the elements from the outside, like rain/snow. You don't want water coming in and getting into your walls. Depending on the outside condition, get rid of the louvres. Ideally you would put a piece of plywood in place and then if you have siding on the rest of your house, put siding over that as well. You might also be able to just leave the lovres there, but seal them up (with caulking outside, spray foam inside) so they don't open and don't let anything in.