I have the 17" Caxton in a 36" vanity. I just checked mine out with a ruler and there's plenty of corner and side room with 3" less counter on both sides.

When we picked our sink out at the plumbing showroom, the salesperson strongly advised us not to get the 15". IMO, 15" sinks look undersized in most vanities. You could mock it up on your kitchen counter and cut the oval out from a grocery bag.

I have the 15x12 Caxton sinks in my master bath 60 inch bathroom. We have no issues with the sink being too small. I don't think they look undersized. We opted for the smaller sinks because I wanted more available counter top space. Here's a couple shots of the sinks. Just divide the pick down the middle and you should be able to judge how a 30 inch vanity looks with the smaller 15 inch Caxton sink.