Troops attack with the normal number of attacks: +1 if double armed or +1 because they are assaulting that turn. All attacks automatically hit a stationary vehicle. Vehicles moving 6" or less in their previous turn are hit on a 4+. Vehicles which moved faster than this are only hit on a 6. WS counts for nothing in this case! Skimmers can only ever be hit on a 6 regardless of whether they moved or not.
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.