Infantry can pose a grave risk to vehicles if they get close enough. With the right weapons, troopers can wreck a vehicle by shooting through vision slits, planting explosive charges on fuel tanks, tearing open hatches to attack crew members or committing some other equally imaginative act of mayhem.
Whilst vehicles can be assaulted by infantry, they cannot be engaged in close combat by them, so when the vehicle's player turn comes around it can simply drive off unless damage prevents it. Alternatively, it could just stand still and fire at the enemy all around it, although this might be dangerous with ordnance or blast weapons!
As mentioned above, vehicles don't take part in close fighting in the same way as units of troops. However, a squad of troops can assault a vehicle and attempt to destroy it. This works as follows: