I don't care if this has already been found, just thought I'd post it for the benefit of those who don't know, because I didn't until today.
The easy way to do vigilante missions is in the following steps.
1. Take a law enforcer vehicle of your choice.
2. Press R3.
3. When you are fairly close to the criminal, pause and unpause.
4. They will step out of the car, and you can wipe them out no problem.
I know what you lot are like, but unless you have something interesting to say don't bother flaming me. Please.