No rotozip bit for glass, a tile bit is not the same. I know there are diamond bits made specifically for DRILLING glass. I don't think there's a jig saw blade for cutting it. Seems I've seen a couple mirrors where they drilled a couple of 2" holes, one above the other to access a square box. If it's not a huge mirror, you could take it to a glass company and get them to cut it for a nominal charge. They'd likely come to you and do it if it's too big to transport. Probably still cheaper than investing in specialty bit(s) and taking the chance of breaking the mirror anyway.......
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