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 where possible and take the shortest route to reach the vehicle. 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.
Launching an Assault
Source: Core Rulebook 3E, p. 90
A squad assaults a vehicle by moving 6" into contact, taking the shortest route and so being unable to swing round to the side or rear unless they start there.