I think you'd be better off just putting a nice framed mirror above the vanity. Then, a cabinet or open shelving (with decorator baskets, etc) to store/hide the essentials.

Tranquil Urban Bath · More Info

I believe you should look for "gangable" medicine cabinets if you want multiple cabinets to work together. I'm planning to do 2 Robern PL series cabinets to make an approx 36" span of med cab above a 40"-ish long vanity in a small bathroom. This involves buying a robern 49.99 ganging kit to combine them (that's the list price, not the price actually paid on the ganging kit). That's the only gangable/modular Robern series I can fit into my budget (comes to $660 including ganging kit, not including tax).

Slightly cheaper option #2 was a Kohler Verdera 2 door, 34" one piece med cab (cost is $440).

I believe the Kohler Catalan line of med cabs are gangable, but too tall for my bathroom and would be pricey like the Roberns anyway.

My hall bathroom currently has a mirror that spans the width of the vanity but it's very impractical (for me) not to have the storage of the medicine cabinets there. I could live with just one medicine cabinet, but I prefer more mirror. But yeah, medicine cabinets can get pricey.

Our medicine cabinet is a triple width all in one piece, from Home Depot. It's about 40" across. You could have a custom cabinet made, though; my GC claims that custom work is not necessarily more expensive than ready-made.

I'd never be without a medicine cabinet in a bathroom! They are the tidiest way to store items constantly in use. Open shelves must be kept totally neat and dusted; otherwise, the room looks messy, and you surrender the privacy of keeping certain items out-of-sight, which a young girl might want to do.

Not suggesting that you have to use Home Depot or a similar big-box store; but you could pay a visit just to clarify what's available. They will have plenty of information as to what kinds of cabinets are available, and they actually talk to you in inches and feet!

I'm assuming that with 66" of wall space for both the vanity and toilet, the vanity's end leg would be up against a side wall. You could either have a large framed mirror or have a mirror run the whole length of the space and put a recessed medicine cabinet in that side wall by the vanity. If you choose to do that, I would have the medicine cabinet the same height as the mirror. Sofia Medicine cabinets are available in 30", 36", and 40" tall.

http://sofiamedicinecabinets.com/

I have an upper cabinet recessed into the wall between the studs so I have a ton of storage in a small space. My mom has a long vanity and medicine cabinet with mirrored doors-she got them from a bed and bath store.