My floating vanity ended up being only 6 inches from the floor. This was because of existing plumbing that i did not want to move.
It did not look like a floating vanity. so the enhance the effect, we added lights under the vanity! so when the lights are on the effect is nice otherwise it does not look to be floating.

I would say 16" will create the separation needed for a true floating look but then your vanity ends being very small for storage...You can do a mock up with some cardboard boxes.

Thanks pharaoh, I do want to do the under cabinet lights as well glad to hear that will help it out. Do you have yours open all the way to the back wall? I was thinking of actually having the toe kick somewhere around 1/2 way back so i don't have a terrible cleaning nightmear all the way to the back wall.

Yes, it is open all the way to the wall. I just use a swiffer to wipe under it.

The vanity is VERY heavy, all constructed from 1" bubinga hardwood. So we had to use come solid bolt to secure it to the studs. The glass counter and marble sink are also very heavy. using a toe kick will significantly reduce the engineering of the mounting...