Huge artworks and mirrors that are leaning would still have to be attached at the top somehow, wouldn't they?? Surely they would be dangerous otherwise, especially in areas prone to earthquakes (even small ones) and in homes with young children. Imagine something that size falling onto someone!

Are there rules of thumb about size and type?

When we lived in Australia, we had exactly the same problem with our children's bathroom. We changed the wall between into a low pony wall, and converted the window behind the toilet into a storage cupboard for toilet paper and other bathroom supplies. We also put in a pocket door like you have and a fresh new shower curtain over the bath/shower. I've attached some photos which I hope helps.

Depending on latches and fixing area, etc - you could get a good glazier with a bit of finesse to custom make a mirror to fit into the window area, and then just centre the vanity below the window. Provided the window glass is obscure, it will still look like a window from the outside, but you'll have an interesting recessed mirror in your bathroom! (hope that makes sense) But, If you want a seperate shower then it looks like you'll have to alter a lot of plumbing anyway, so you might as well move the vanity while you're at it.