You need a special tool. Essentially a "hook". It must be at least six inches long, have a handle on one end, a thin, straight steel rod with a hook on the other end. You must carefully reach down into the vent approximately four inches and hook the very back end of the vent. Reach down near the outer edge of the inside at the 3 O'clock position (not near the center) because you are hooking on the outer edge of the vent at the rear. You need to pull straight at you with a considerable amount of force. Do this at the 9 O'clock position, then at the 3 O'clock position, alternatively. Ideally you would have two similar tools and can pull from both the 9 and 3 O'clock positions simultaneously.
The dealer used a tool that looks almost exactly like this: Anglers' Choice T-Bar Hook Remover | Academy
He popped one of my vents out, handed me the tool and I popped another one of my vents out on my own, in about 10 seconds.
Removing the outer ring first gets you nothing because if you try to hook into the access holes behind it, you are essentially also hooking behind the steel ring behind the vent that is permanently attached to the dash. You'll pull the dash out before you pull the vent out if you try this. Removing the inner vent assembly does you no good either because you can easily get the tool past the vents and hook into the very back end of the vent without removing the inner assembly. Under no circumstances should you pull against the back of the inner vent slats themselves as they are way too delicate for the force needed to remove the vent. The "tabs" that others refer to are at the 2, 5, 7 and 10 O'clock positions and while they keep the vent from popping out on its own, do not hold the vent in such that they will break if you try to pull the vent out without depressing these tabs, so don't worry about these.
So, to review, you need to hook behind the very back of the vent (the vent is 4" deep so you need a tool at least 6" long) and pull straight out toward you.