I have to agree with the vigilante technique. I've dealt with the cops before when it comes to stolen property, and they really won't do anything unless you can PROVE ownership, and for them, that means clerical proof that you purchased that exact moped (that's why bike shops are supposed to write the bicycle serial number located under the bottom bracket on the sales receipt).
Without written proof that you own THAT moped, and not just _"see officer, I made that scratch there"_ or _"mine's the only moby around with white grips and a test-tube of oil in the storage compartment"_ they really can't do anything.
In fact, if you go to them, they're going to tell you that unless you can prove ownership, the moped belongs to the accused; possession is 9/10's of the law. So, if you go to the cops, this is what they're going to tell you, and then WHEN we go and take back the moby, WE will be the thief's, and the person who took Tim's moped can then call the cops and say _"remember those knuckleheads who accused me of taking their moped the other day? Well, they just came and stole it."_
So, how about we go to Home Despot and grab some hardened steel chain and a GOOD lock, and stealth in there and lock up the moped next time we see it. Then we have time to swarm without worrying about it being gone when we go to destroy.