I think the line becomes blurred when you actively gather the evidence, rather than happen upon it, and post the videos online to publicly shame the people in question.
So if a a cyclist decided to bike up and film people smoking marijuana in their car, then submitted it to police aggressively, you wouldnt feel it was a vigilante act?
In a jurisdiction where driving while intoxicated on MJ is illegal, no, that's not vigilantism. You definitely don't want people driving around stoned in London traffic conditions - very much need to keep your wits about you when driving here.
Yes, peoples' reaction to and tolerance of weed varies considerably, but there's just no need or reason to drive while high here.
