Grand Theft Auto: San Andreas - Vehicle Missions Guide

Entering a certain vehicle and pressing a certain button will activate a certain mission.  These missions can unlock very useful rewards, if you wish to remain cheat-free to complete the story or just goof off.

Vigilante -        150% armor

Entering a police vehicle, such as the police car, police bike, tank, S.W.A.T. tank, or Hunter helicopter will grant access to these missions.  
The objective is to kill the criminals, who will be in vehicles and will not exit unless it catches fire.  Upon doing so, they may attempt to kill you or carjack someone.  It starts with one vehicle and, the maximum, as far as I have seen, is three.  You are timed, but gain time for each level you complete.  

12 levels gives you the reward.
The Hunter helicopter makes it easier to complete these missions, one can either be spawned using a cheat or found at the airstrip after the pilot school is completed with all gold medals.
The desert is an easy place to complete them.

Paramedic -            150% health

Entering an ambulance grants access to these missions.  You must drive around and pick up wounded patients and transport them back to the hospital.  You are timed but are given more time for picking up and dropping off patients.  Be careful not to run any over or destroy your ambulance.  The timer also resets to one minute every level.  

12 levels are required for the reward.
Doing these in Angel Pine is much easier.  

Firefighter -         Fireproof

Entering a firetruck will grant access to these missions.  You must spray the burning vehicles and the pedestrians that exit them before the time runs out.  You will be given more time for completing levels.  

12 levels are required for the reward, taking no damage from fire.
People say Angel Pine is easiest, but I found it difficult due to the firetruck's tendency to tip over.
I did the missions in San Fierro.  I failed once because I was unable to get somewhere the first level, but restarted it and completed all twelve with ease.

Taxi -            All taxis have nitrous oxide and can jump

Enter a taxi or cabbie to grant access to these missions.  Just pick someone up and drop them off wherever they want.

50 levels, though not all at once, must be completed.
Enter the flying cars cheat and use the more health cheat often to complete quickly.

Pimping -               Prostitutes pay you

Entering a Broadway activates the mission.  One can be found parked beside a gas station not far from Ganton.  Simply pick up and drop off girls wherever the game tells you.  Sometimes you have to kill trouble-makers.  Only when you are dropping off a girl or killing a trouble-maker do you have a timer.

10 levels required for the rather useless reward.
They can only be completed in a city, activation in a town forces you to drive to a city.

Freight -             $50,000

Enter a train to access this mission.  You could wait around a train station for one to appear, they normally do soon.  Simply drive to a station and stop in the designated area.

Get to a higher speed before starting to make the first level easy.
Keep your speed at or below 45 and around 40 on some turns, just to be safe.  
Depending on your time, slowly stop in the area, or at around 250 - 350 distance, begin braking quickly.  
The cinematic camera can be frustrating at times.