Assuming the medicine cabinet is centered above the back of the vanity, I think the same sconce on each side would look better than one.

What are your reasons for only using only one? It is a stunning statement lamp - does it serve your function?

You are aware of shadowing from above. Be advised there is shadowing from the sides as well.

Is it a design no no? Probably. Both from a form and function standpoint.

If you have a light on only one side, you're going to have the same problem as with a light above--just from the side instead of the top.

What's the reason for only one side?

there is no room on the other side as there is a wall.

You could mount one on that side wall (they won't be symmetrical, but...)