Ohhhhhh, he/she is right! I don't have a vessel sink but my new glass sink doesn't have an overflow. First, I had to purchase ($48) a new drain fitting. It's directly under the sink and doesn't have the holes at the top like the ones for a sink with an overflow. Right off I noticed it was draining much slower and asked the plumber about it. I thought there might be some trash in the drain but he said that without the overflow, the sink will drain much slower. There are "vents" at the top of the drain in a sink with an over flow. I guess the air aids in drainage. I don't like it but it's not a deal breaker and I would have still bought the sink if I knew ahead of time.
Monica

I have a vessel with an overflow.
Donna

If you choose a vessel sink with no overflow, then there is a special air vent that you incorporate into the drain line. They work great. You just have to be sure you have room inside the cabinet to accomodate the vent because it has to be higher than the drain line. They are available at any good plumbing shop.

green-zeus, thank you for this information. I have an undermount sink but it has no overflow and I'd love to be able to use what you described to help with water flow.
Monica

I think one of the proper terms for this is a "vacuum breaker." I helped a friend completely remodel her half bath a few years ago and the plumbing place where she bought the vessel sink warned us about this so we bought the vent with the sink.

I'm sure if you do a search on a plumbing site, you'll find this. You install it ahead of the P-trap.