The vanity in my hall bath is 96”. The mirrors are 24” wide. I used 3 sconces. I also have 3 four inch recessed can lights in the ceiling, as well as a can light above the tub/shower.

I think it mostly depends on what you like. Having two framed mirrors does help the space look more updated, in my opinion. But if you decide to keep the full mirror, you can always add a frame. However, if you are replacing a 64" vanity with a 84" vanity, then the mirror will no longer be the same width as the vanity, right? That's fine, but it won't appear like your run-of-the-mill builder-grade mirror anymore. That might alter the look a bit and its hard to say if it'll still look good without seeing it. Can you upload some pictures? Regarding the sconces, I think 3 could work.