In addition to the dimensions of the mirror and the vanity width, you should bring into consideration:
- Will there be 2 people sharing the bath, or is this a single person bathroom? (If 2, I'd hang horizontally.)
- Vanity and ceiling height? Is there a high backsplash?
- Is it a high faucet?
- Are you particulary tall?

My first thought was definitely hang it horizontally, until I did a mockup. Now I like it vertical, as it seems to give the room more height. Of course this is just a simple mockup. (Squares below are 3" scale. Sizing approximate since PowerPoint is not exactly engineering-worthy.)

If you hang vertically, you have room for display shelves on either side.

Try taping up a square of paper the same size as the mirror and see what you think.

{{gwi:1396089}}

Thanks, I think I like the looks of the vertical.......gives some height to the room.