Since the game simply puts up a notification stating the number of dead guards, I doubt that the game was keeping track of what killed them. More likely is that is was just keeping track of whether they were alive or dead. This would be the most efficient route since the programmers can assume that the guards will never encounter any deadly situation that isn't caused by the player in some way. Even if they do, if the goal is to make dead guards, then the thing that makes the guards dead isn't going to be very important.

However, if they were to make a system that specifically mentioned the player being the one to kill them with a vehicle, they could do so by setting up some tagging system along these lines:

• Player sets C4; C4 belongs to player.
• Neutral vehicle interacts with player-owned C4(explosion); Neutral vehicle now belongs to player.
• Player-owned vehicle causes lethal damage to guard; Guard killed by player.