It is difficult to get a standard sized (good sized) sink into an 18" cabinet and still meet code (if the sinks are between walls). You must have 15" between center of the drain of a sink, and the wall.

Also, many good sized sinks (undermount) are about 20" wide. That won't fit into a 15 or 18" cabinet.

So, 21 is about the minimum width, and you'll have to be very careful which sink you choose, as well as placement.

If your vanity is between 2 walls (so, there are walls on the 2 ends, and not open space), 24" is the smallest you should plan to go.