I also have a window like that in our shower - it may, but doesn't normally get wet from the shower. I have put in a thin venetian blind in white (made from plastic) very cheap, under $5. I am in process of making an puffy austrian blind to hang above the window, for a bit of colour I am making it from a shower curtain). It will be there for looks, I may never pull it down. The venetian gives privacy and blocks the light a certain amount.

Is your final photo of the vanity you already have? I also thought adding legs would take it back to the MCM you are after. Would be interesting to know how the silver drawer pull is attached. Not sure about painting it matte black as I thought timber furnishings were more prevalent then, but it's totally your choice. I hope you will post updates as you go. I just love mid-century.

depending on the size of the existing vanity it should be possible to have a similar new vanity or a custom unit made to replace the original but a chat with a plumber should be arranged to recommend suitable options and plan a schedule to remove the old and install the new, including any necessary waterproofing or tile repairs..btw careful removal could allow the original sink, taps and even the counter to be saved and recycled into a new vanity that matches the bathroom perfectly with the added option of being able to add different doors or drawers to the cabinet

I did a cavity door to my small bathroom. I reused the existing door and left the existing door handle. It was a beautiful door and that way stayed within the look of the rest of the house.