Depends which walls they're in, Thunders...........
Interior partition?
External wall (interior)?
External wall (exterior)?
Also, what finish is there now?
Ah ......... if there's a solid fuel type device ...... woodburner etc, the vent may be for that.
Otherwise, interior, small piece of plasterboard, plaster in, or polyfilla and paint. You could leave the outside vents alone then.
Also .......... offcut of Celotex/Kingspan pushed in for insulation ........ or standard loftroll. Put the insulation in a plastic bag to isolate it from the outer leaf.
The fire places have been long bricked up but stupidly it is the two rooms with no fireplaces that have vents, i have a nice efficient combi boiler, was put in a year before i bought it. So basically I knock out the grill stuff some insulation in and put plaster board over it and then skim it up to wall level