A squad can assault a vehicle by moving 6" towards it, getting into contact in the same way as when assaulting enemy squads. Individual models must move into contact with the closest aspect moving by the shortest route. This means that troops are not allowed to go round the sides or rear of the vehicle unless they start their assault at the sides or the rear.
Diagram: "Assaulting a Vehicle" — a Space Marine squad launches an assault against a Leman Russ tank. As they begin their assault within the vehicle's side arc, they must attack against its side and use the vehicle's side Armour Value when working out Armour Penetration.