Or you can get out and use the grenade launcher. The money flies out and you are gone before they know what happened. Honestly....I like finding the guys that are walking to the armored truck and run past him and steal the truck! One time he screamed..."what the hell!?"
I just found this out by accident. Drive into the water. I killed the guards before they got in the truck and stole it. To lose the cops, I was driving in the reservoir. I lost them but drove into water that was too deep (yes I'm that good). The back of the truck opened and the cash was lying right next to it.