I don't think there is much you can do about the vent, but I would put a chair with a floor lamp (you can use the one in the picture) and maybe a small side table on the right side of the fire place. Also I would make the drapery rod over the large window come out past the edge of the window, so the curtain covers part of the window and the wall.

Paint vent same color as wall. Remove TV from over FP, it is way too high & looks awful. If you can't move it to another wall lower it to the top of the mantle & do not decorate mantle. Raise curtain rod so that it is over the entire window. Post pictures of the other walls so more suggestions can be made.

What an odd place to put a large vent. I agree with the other comment, that you should paint it the same color as the wall. Aside from that, you could get an estimate on what it would cost to move it higher on the wall. If it's up higher, you could do a nice built-in.

I think the TV is fine over the fireplace, but I would lower it and make sure it is centered (looks off center in the picture).

Thanks for the advice everyone, we can't move the TV to another wall. Here's the other side of the room, ( sorry I used the listing picture, since I'm not home and the picture is before the room was painted) but on the other side of the room there are windows.