I did this once and put thin molding between each mirror, which gave it a french door look. I would use plywood for the backing.

We're nearly twins, jerzee! My vanity was supposed to be a warm mid-brown and it turned out darker, more mahogany-looking than I'd planned. Counter is also Cararra. My walls are SW Antique White, sort of french vanilla, and floors are a mid gray tile. Initially I thought the walls were too warm for the grays and chrome, but I like gray and cream together and I'm leaving it all as is. I had stressed over matching or mixing the metals and, in the end, have mostly chrome except for a bit of satin nickel. Nobody notices or cares. I hope you like the original finish on the mirror. A lot depends on how it works in your space. I have plenty of natural light in my bathroom. If you decide to go more silver, I can share what I did, plus what I did at first and what you should avoid doing. :)

Don't have a photo at the moment but it is Keraben moldura petra bone chair rail. Several online retailers have it listed but I contacted them and they said they don't actually have it. Drove to Rhode Island to buy the last 2 boxes I could find.

Yea to your remodel! And, yes, mirrors are never out of date. The key is installation particularly transitions from mirror to adjacent surfaces. As to what height to end the tile, a good 9-11" (44"-35") above the vanity is about right in my opinion. The biggest problem with mirrors is the splashing of water from working at your sink so the 10" "backsplash" is appropriate - don't want to have to constantly clean those mirrors.