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Advice for drive dying would be to try and hit the cars gas tank which will save ammo, time and wipe out everyone in the vehicle in one go. Keep this up and kill all of the required criminals to beat the missions. The map will guide you to the general location of the Snapshot, but you can also refer to the text descriptions and screenshots for clarification.
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When all snapshots have been taken you will earn some cash and a respawning weapon cache will then appear outside of the garage in Doherty. If you're heavily wanted, ditch the cop vehicle, grab a normal one, get it sprayed then get back in the vigilante vehicle and continue. There are 50 Snapshot locations in San Fierro. Get to know the map, know where Ammu-Nation is if you need some ammo, and know where the Pay N' Spray's are. My advice would be to go for the missions in a city such as San Fierro. In the countryside you'll have more chances to lose the cops however, but the distance and terrain you'll have to cross to get to the criminals can take away the advantage that it may bring. Despite the fact you're actually helping the police with their work, they dislike anyone killing anyone unless they're responsible, and after a few levels you'll have a two or three star wanted level which isn't too good. One of the problems with this missions is the amount of heat you will get onto you. In the countryside you have more opportunity to take shortcuts across fields and what not, however that also increases the chances of crashing and wrecking your vehicles too.
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There doesn't seem to be a specifically easy place to do these missions, unlike the previous two covered, however they seem to be relatively easy wherever they are attempted.
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Now like all "Sub-Missions" I suggest you do the Vigilante Missions as soon as possible, as the rewards you earn for completing them will help greatly in the story or just gameplay in general.
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