Wall- use mirrors to reflect what you want to see, it will provide more light and hopefully the illusion of space. Can you put it on landscape orientation on the wall?

How about a large frameless mirror and sconces each side? Or even mounted on the mirror like this.

Were you meaning to add this photo from your ideabook? If not, I'll can go back and delete this reply.

with a wall cabinet almost 4 m wide i would expect to have at least 9 doors each over 440mm wide?? and with only 3 doors or even three pairs of doors you would have to step backwards to open the doors and they're much too wide and heavy ...how deep is the vanity below the wall cabinet and is the wall cabinet recessed into the wall or wall mounted?... do you need this much wall storage for medicines and toiletries? combined with storage in the vanity an average wall cabinet 1200 wide (with three doors) should be more than enough for the usual collection for one person ...consider a recessed wall cabinet say 2400 wide with six doors and the balance of the wall with fixed mirrors ..you need to talk to the builder and/or cabinet maker to decide what suits your needs