There are sure a lot of painted vanities in photos. Lots are being sold online. I've never heard that it's a problem.

My childhood home had a big linen closet and painted door and window casings. They were fine -- but they were not in direct contact with water as a vanity might be.

I've had painted vanities my whole life and never had an issue.

The reason I ask is that when we redid our kitchen, cabinet company sales people were telling us that the door finishes on painted cabinets may separate at the mitered corners when the wood expands and contracts due to fluctuations in a home's humidity. At least it would be more noticeable than it would be on stained cabinets. Seems the humidity would be even more of an issue in a bathroom.

I had painted vanities put in when we built our house 30 years ago. The paint was cracking and peeling within 5 years. I would not do it again in a bathroom. But I think it depends on the quality of the finishing, the qualifty of the venting, and how careful you are to avoid drips from the sink.

I've read here that mitered corners are the worst for expansion/contraction and separation. I have regular shaker (painted) in the kitchen and the faint line where there is some movement is insignificant.