How do you get vigilante missions in gta4

To carry out vigilante work with a cop car on GTA IV (PS3):
Jack cop car.
Park up in convenient place.
Press L1 to access cop computer.
View Current Crimes: press to choose job.
When job is chosen, red marker will appear on map and route will be shown, with timer on top of screen showing how much time you have to get Niko to the crime scene. If he does not get there in time, a failure notice will appear on the bottom of the screen. If he achieves the crime scene on time, follow the instructions and clean up the crime scene as required. If any cops in the area see Niko breaking the law, they will go after him, even if he is doing vigilante jobs. Any wanted stars would clear after a crime scene is cleaned up.

To add to 100% completion of the game, achieve 20 of these vigilante crime scene clean ups. Niko can continue to do these vigilante jobs even after the 20 have been achieved.

View Most Wanted: Similarly, choose the criminal to be eliminated. Achieve all of these (30) to add to the 100% completion of the game. Once these criminals are wiped out, this cannot be replayed.