1) At least half the time when I type the phrase "vessel sinks" I mistype it "vessel dinks."
And, more importantly:
2) You need a special kind of drain for them(sinks, not dinks. I don't want to know what kind of drain you need for vessel dinks.) A normal pop up drain (the image on the left) won't do, as a vessel sink (usually) has no overflow. Most building codes require a grid drain instead, which has slots out of the top and is 1/8" larger than a standard pop-up drain. Okay, maybe you did know that, but I certainly didn't, and wouldn't have, if it were not for the help of David Smith, of SmythCid Pottery, where they've forgotten more about stoneware sinks than I will ever know. Check out some of their sinks here.
After shrinkage the measurement of the lip would now be 2.5".
A thing that I did know, if I thought about it, is that the edge of the drain must sit below the bowl, or an icky little pool of water will collect at the bottom. I knew it, because I know that water can't flow up; but would I have thought of it before or after I fired the first sink? I am spared finding out, thankfully, since David pointed this consideration out in his email in addition to the drain information.
So! Off to reconfigure the drain hole on my first sink. I had not yet made the hole in the second.