I worked the LA area. You want to see a gang riddled area... try Lennox, East Inglewood, Compton along Rosecrans, Watts. It is obvious and rampant. I know other cities have this as well.
From a gang standpoint...you get hassled if you look "ganster" and wear something that distinguishes you as being affiliated to another gang.
If you see a person who looks like a gangster, has tats like a gangster, talks and has mannerisms like a gangter...then he/she either is or wishes he or she was in a gang. (If it looks like a duck...)
They get treated as gang members by gang members and police. If they want to avoid the stigma, then dress and behave differently. Obviously, if they aren't affiliated, then they enjoy the attention, or they wouldn't dress and act that way.
Police can, with a concerted effort, signifantly impact a problem neighborhood with parole searches, task forces, undercover operations, stings, etc. Presence alone doesn't act as a deterrant...just a temporary delay in activity around the cop and cop car.
I think the vigilante approach isn't necessarily bad. But it is upon the neighborhood to make the calls, go to city hall meetings, demand better response time and response results. The vigilante approach caused attention to come to the problem...and that is the goal.