Tanks and armored vehivles are the backbone of your army's power. It would seem that you should never, under any circumstances, abandon your vehicles, but in reality, leaving them is sometimes the smart idea. Evacuating a vehicle is very simple - just press E to order the crew to disembark; the type of the unit you're controlling will change. If you have selected a few tanks, of which some are empty, everyone will evacuate. Alternatively, you can select a single vehicle and click RMB on any of the crewmembers to order them to disembark.
When evacuating, remember that the main goal here is to protect the drivers. The AI will never destroy or take over your abandoned vehicles during the campaign, but such a vehicle is still susceptible to stray projectiles fired by tanks, artillery or cannons. After disembarking, get away from the vehicle ASAP; cease fire and retreat to safety. When you clear the area, you may come back to reclaim your vehicle, mend it, rearm and refuel, hence making it battle-ready.
This strategy may seem problematic at first and it won't always be effective in a tough situation. But you may benefit greatly if you're lucky - you will save the crew and vehicles to fight another day. There are some situations in which it is highly advisable to abandon your vehicles:
- Running out of fuel or ammo. This is pretty obvious. If you don't have any cargo trucks, best use the crew as additional infantry. If you have few resources, you should select only a couple vehicles that you're able to fully arm and fuel, rather than use everything you've got just to stop in your tracks middle way.
- Having no vision. If you've lost your infantry, but still have considerable armor power, order one driver to disembark and start crawling towards enemy lines. You will gain vision that will allow your tanks to lay down effective fire.
- Outnumbered. If the incoming enemy force is huge, and your vehicles are either too slow or heavily damaged to retreat, order the crew to disembark. Even if you lose your men, the vehicle will survive. This is particularly advisable in case of rare or poorly-armored units (heavy tanks, destroyers or rocket artillery).
- Critical damage. If your tank gets disabled and you still have incoming enemy, just leave the vehicle to avoid having it destroyed. The crew can come back and wait for repairs.
- Extremely low HP. When your tank has less than 10-20 points, practically every bullet from rear or from the sides can destroy it (even from the front, if the armor's not too thick). You should rather evacuate the crew and use them to man another vehicle, instead of letting them die a pointless death inside a metal can. Only the support doctrine will allow you to save such a vehicle, because it allows to instantly replenish 50% of its health (it's still better to do that after securing the area).
- Too many vehicles, to little infantry. You need infantry for almost any sort of offensive. If you have too many tanks/cannons/vehicles and you're taking over the vehicles abandoned by the enemy, you may run out of infantry needed for recon and offensive maneuvers. Consider leaving some light tanks and vehicles at the back and creating a unit that consists of their crews. They're not the most effective force on the ground, but they can create diversion and provoke the enemy to reveal themselves. Should such need arise, your soldiers can retreat and reclaim their vehicles.