We already have sharia patrols in parts of londonistan - isn't that a form of vigilantism? If the authorities don't want vigilante patrols then they need to get a grip of this problem and quickly, at the moment I'd say the Islamists were way ahead of the police in terms of competence.
I think there were a couple of idiots who were going around Whitechapel at night threatening gays/people drinking/basically anybody, but I think the police cracked down on them pretty hard, (I think they were all arrested, possibly imprisoned)
For what it's worth, I walk through Whitechapel twice a day to get into the city for work, it's JAMMED with Muslims, but I've never felt threatened or upset in the slightest.. When I get to London bridge however, I get shouted at by a load of crazy christians handing out creationist leaflets, so I prefer Whitechapel at the moment..