Hanging a shelf on a difficult wall
You might be able to use an "EZ Anchor" (that is a brand name but there are other makers). They are only about 1 1/4" long and you can even shorten them by cutting off the tip if needed, to accommodate for restricted clearance behind the wall.
They hold plenty of weight for lightweight shelving and knick-knack type stuff (I would not use them to support a TV).

So... 1/2" for the drywall, a bit for the thickness of the shelving unit, plus an inch of space between wall and vent, they should work perfectly and you can get them at almost any hardware store. Please shop at your local mom-and-pop store and buy American made products when possible! :)