Hi does anybody know how to make vehicles destructible, as in you can blow them up and they will explode and be unusable. like in h2 coagulation:

http://www.youtube.com/watch?v=lBOkqBbyZeQ

You should have two models for the vehicle: one usable and one destroyed. Then you edit it's health and explosion effects.
Vehicle health is not communicated over the internet... when you join, all vehicles will be at 100% (even if they are damaged from the player that damaged them before you joined). So a vehicle could be dead at some point, but look only a little damaged on your screen.
People don't like this because you'll end up trying to get into a dead vehicle. It is advised that you don't make destroyable vehicle in MULTI-player maps.