Fix subflooring near wall with ventilation
Cut the opening around the old hole 2-3" larger than the hole that is there now, unless you can cut an edge in the middle of the joist. With that opening large enough away from the metal in the hole, set halfway under the newly cut edge, 2X4 placed flat so that approx. 1 3/4" is under the original floor and the rest is out in the opening. Glue this in place and draw in place with screws. 

Cut a new piece of subfloor and screw and glue it in place too. The blocks you cut only need to be a few inches longer than the hole on either end. It will be stronger than the original. 

On the side that is close to the wall you may not be able to cut 2 or more inches, if you can leave 1 1/2" of subfloor there, it may be tight to get the block in, but you will still be able to cut it with a circular saw. If you have a reciprocating saw or multimaster you could go as close as an inch, and still get screws in.