I would think the mounting ring is required, not for the seal but to provide a smooth, even surface for the glass to sit on. In the one glass vessel sink we have, the ring adds only about 1/4 inch in height, so height shouldn't be an issue. The ring sits on the counter, and a rubber washer sits on the ring and supports the glass sink; I doubt that would be as stable if the rubber washer just sat on the counter. Looking at it now, I think it would also be harder to clean around the base of the sink if the glass sat directly on the countertop. (As for the seal, at least in the case of the one I installed, that takes place inside the sink; the center drain is sealed to the sink with plumbers putty or a washer and then tightened from under the countertop with a nut around the drain tube.) Here is a link to installation instructions for the Ronbow glass sink I have, if it helps you better visualize how the pieces work together. I would strongly suggest contacting the sink manufacturer you're planning to buy from to see whether they say the ring is optional or required, but my sense is that you need it.

For what it's worth, I love the look of the vessel sink we installed in our main floor powder room, but I wouldn't be happy with vessel sinks for our master bathroom. The vessel sink is pretty as a work of art, but isn't as practical for daily use and for cleaning as a larger, undercounter sink in my experience. Obviously, people disagree with that and install vessel sinks in master baths; I'm just throwing out my opinion.

-- Eric