Yes, I had noticed a lot of pics on Houzz, that's what gave me the idea! I certainly wouldn't put it IN the shower! We have a tiled shower, but the tiles only go as high as the glass surrounds, so there is about 30cm of painted wall above the shower, which is the part I was worried about with steam etc.

We have a 900x900 shower and it's plenty big enough. Neither of your proposed layouts looks that great, to be perfectly honest, sorry! In the first layout, you could improve it quite a lot by swapping the toilet and vanity (centre the toilet under the window on the bottom right of the picture if possible), and having the shower door on the other wall (beside the door). Then, you could have a towel rail on the wall beside the door (i.e. behind the door when it's open), which would be accessible from the shower but hidden when the door was open. An 800x1200 shower might be a good size for your space. You'd have to choose a toilet that doesn't protrude too far, plus a narrow vanity. With that layout, you could have a long but narrow wall-hung vanity, with a big mirror covering the wall between the windows, which would make the room feel bigger and would give a good amount of storage. Also, you may find this helpful: http://www.houseplanshelper.com/small-bathroom-floor-plans.html?utm_content=buffer4aa20&utm_medium=social&utm_source=facebook.com&utm_campaign=buffer

Maybe look beyond the footprint of existing bathroom. We renovated a bathroom where we were able to have a seperate large shower and keep the bath by incorporating a built in cupboard in a bedroom room adjacent to the bathroom. New Free standing storage in that room has resolved any issue with the loss of the built in.

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.