Those plans don't even sound good though even if the vigilantes thing is a good idea. You can't chase thieves on a vehicle while you're on foot, and please don't get mopeds of your own to chase them because a vehicle chase of random people in London will get people killed.

If people do plan on doing something themselves then it's just gonna be a lot of waiting around. You'd need 5 people, go to a road that's a hotspot for moped crime, get a guy to stand on each corner of the road and the last person(preferably a lady) in the middle as the main lookout and to have a cheap phone out so she can also be bait. Then as soon as a moped thief takes a phone everybody would be alerted right away and the guys at the ends of the road will have time to see the moped coming and attempt to knock them off. As soon as they're off then it's the easy part, 5 people + bystanders to hold them down until police arrive. Of course you make no mention to the police that you planned this, just say you saw it happen by chance.

Most days will result in nothing happening and you'd be standing around for 5 hours a day without seeing a thing, just saying.