After you complete "Crime and Punishment" you will have access to police computers. When you enter a police car, and are parked, press to open up the police's computer. In there you will see a menu to "View Current Crimes." This is going to be your classic Vigilante mission that we've all grown to love. The best part is, they no longer have to be done all at once, you can take your merry time and do them at your leisure.
When you select a current crime on your computer, a red GPS beaon will appear on your map taking you to the offender. Eliminate them, and then repeat with 19 more offenders of peace and justice.
If your police car becomes damaged, here is an easy way to get a new one. Choose an offender to go after. Quickly head to the crime scene, as you're timed on how fast you get there. Once you arrive, you can take your time, so as soon as you arrive at the scene, start wreaking havoc until police arrive to stop you. Kill both cops that come out of the car, then kill the offender. As soon as you complete your mission, your wanted level automatically disappears, so you can go and take the freshly slain cops' car free of charge.
As a note, the computer will tell you what type of a crime you will be going after. Criminal on foot will be the easiest, up to a stolen car chase will take the longest.
**Can be done after completing story mode**