Good question. I've also wondered about this, but so far all my steam showers have had lighting as part of the steam unit package from the store.
I know Lotus makes this one, but it's also pretty expensive. You have to buy the correct trim kit for it to be workable for steam:
https://www.lotusledlights.com/produ...cessed-led-15w
As far as vapor-proof: Is that an actual UL listing standard, or just a marketing tagline?
I know many of the better LED puck lights are listed air-tight and wet location rated. I've thought about setting up a test with those siliconed inside a steam box, and see how much pressure they can take, but....who's got the time?!!