in my vanity, I have a 22" center section with the sink being 19-1/4" wide. So that means you could put a 14" or 15" sink in the 18" center section IMHO. That's 1-1/2" on each side for my setup. My cabinet is frameless and I don't know if that makes a difference. I wouldn't think so.

For example this Kohler Verticyl round sink is 13-3/4" diameter, and should fit in an 18" cabinet, using my logic above:
Kohler round Verticyl bathroom sink

I have the oval one and like it fine. Its contemporary. But I am sure you can get what you need from Kohler and other companies to fit your 18" opening. I'd go for the 18" center, rather than the larger center. I think the larger center will make the side draws too narrow.

Here's my vanity to help with scale. Its 60" wide. Center is 22" and the side drawers are 19". That's about the same ratio your vanity would be with the 18" center.

The Verticyl round would fit in an 18" cabinet, but I think a 13" sink would look undersized in a 48" vanity. A 13" sink is better suited for a powder room where it would only be used for hand-washing. Kohler has all their spec sheets on their website. I looked at their smaller rectangular and oval sinks and didn't see one that would fit in an 18" cabinet. Looks like they discontinued the 15" oval Caxton.

But I would not want 12" drawers, especially only two of them. I could not make use of three cabinet door bases in a vanity because I have more small items than large items, which I'd guess is typical in a bathroom. You could choose a design that has all drawers for the sink base, with a standard 24" sink base. Your top side drawers would be useful for a comb, brush, and dental stuff. Otherwise with an 18" sink base, you'll have to decide whether you want to compromise on the usefulness of the sink or of the drawer storage.

Thanks!

I figured out by talking with designer #1 that the dimensions of the 17 x 14 kohler 2210 sink, at its widest point includes what she called the flange, which lies "flat" against the underside of the quartz vanity top. This is what makes it 19.25 at the largest point. The part of the bowl that goes down into the base is 17 inches wide at its widest, and she was not at all concerned about it fitting. I can only assume that this will fit based on that.

So then I went to the counter fabricator and they feel that that kohler sink will fit, but they will know for sure when they come to template. In that case, if it does not fit, they would use the the smaller 15 x 12 Kohler 2209 (17" at that largest undermount area).

I looked at the sink while there. So I think it just depends on how high the draw box itself is to the top of the cabinet...and I would imagine the draw box wouldn't be flush with the top of the cabinet.

I have an old style top mount sink so I measured my current sink opening and it is 15 x 10 !! So even the smallest sink i mentioned above is bigger than my current, so now I'm no longer worried about that! Whew, that's a win! LOL