Why won't Vigilante sub-mission end at level 12 like others?
In GTA San Andreas the vigilante missions are infinite but after completing level 12 they stay the same difficulty and you get a body armor capacity bonus.
Unfortunately the armor bar is not updated, so if you miss the message about the armor bonus you might not notice the difference. To see if you have unlocked the bonus, all you need to do is fill up your armor bar than go to Stats / Player and scroll down to armor it should display 150.