I have a small mirror that fits on an adjustable arm in a slot on my Bell Metro helmet, also on the Citi, I think, but it sits folded back for emergencies or riding in city traffic, which I rarely do.
Normally I use bar-end mirrors on all my bikes. Two that I know from experience to be vibration-proof are:
1. Zefal DoobackExpensive but good, except it got regulary smashed because it made the bike too wide for a footbridge I like to take at speed.
2. Cateye 300GCheap (mine was a fiver), not vulnerable to breakages because when knocked it folds away under the handlebar (if you mount it thoughtfully), a scaled rear view because it is not domed at all, so you can actually tell how far the car is behind you. Mine is mounted to sit under my hand, where it gives me the widest space for my hand on the shortened grip which allows for the Rohloff control. Highly recommended.
3. Chinese no-nameThere's actually a third round mirror, similar to the Cateye but with a domed mirror, which I found has a vibration-proof mounting, but I gave it away and now can't remember its name. In any event, I can't recommend it because it would show cars a long way back and then, when you turned into the lane, you discovered they were actually already passing you, very dangerous distortion.