I had to drill a 10" hole through the side of an air traffic control tower. I did this:
Get a good size hammer drill and a 1" masonry bit long enough to go through the wall. Draw a circle where you want the hole. Draw a circle 1/2 inside of that circle. Drill around the inner circle ever two inches. After all the holes are drilled you use a stone chisel in the hammer drill to clean the webs out. Once every other web is gone, a few smacks with a big maul will dislodge the chunk of concrete. If you find rebar that you need to remove, you will have to cut it with a long demolition carbide blade in a reciprocating saw. Clean up the hole with the chisel bit or a stone hammer.
If it is just for ventilation and air exhaust, you can just drill a lot of 1" holes in a honeycomb pattern. No need to remove the concrete plug or cut any rebar.
Fit a cover as needed on the outside. Lot of drilling, but would work.