Ikea has a very narrow vanity available with a few different door options (and you could look into custom doors as well, Scherr's makes doors for Ikea products)

Sonia makes some very narrow vanities. They are pricy though (we had someone local make a "copy" of one of their vanities for us for much less $).

Here is a link that might be useful: Sonia

You can get standard sink bases in as narrow a depth as 16".

Here is a link that might be useful: Kraftmaid

I saw some on the same site I got my decorative hardware from, www.cabinetknob.com.

Here is a link that might be useful: Builders Resources

or.....you can think out of the box and look at other things that would substitute as a vanity.

A narrow but tall nightstand, maybe?

Or, as I did, solid wood microwave cart with cabinet storage and drawer.

We had a tiny powder room that had a very shallow place for the vanity/sink. I found a night table with drawers, and as we were using a small vessel sink, just put the appropriate holes in the top for the drain and faucet and voila!

There is a thread right now describing exactly what we did with the drawer to accommodate the drain pipe in the middle- except it's Mongo and it looks much prettier.

When we get a chance, we will swap out the wood top for a piece of stone... not a priority on our list of "to do."

Here is a link that might be useful: Drawers and pipes