Bathroom Mirror Replacement

We have a large, single pane Mirror over double vanity sinks. that goes wall to all with only 4" between the top of the mirror and the ceiling. We need more light and as-is, there is not enough room to add vanity/sconces above. While the mirror is not exactly pretty, I don't feel the need to replace the entire thing if I can avoid it. Can a mirror be cut while in place or do we need to rip out the whole thing and just do a complete replacement since we plan to add lighting as well? Where do I go for just bathroom mirror replacement like this without being reamed by someone? I don't mind doing a little of my own CM work, I have a painter and electrician already so I have the other guys to fill out the work.