A vehicle can move up to its full cruising speed (=12") and still disembark passengers. In general, a passengers cannot disembark if the vehicle moves over 12", although there are some special codex-specific items that are exceptions to this rule. This is from p70 of your rulebook.
Passengers may not embark NOR disembark from a vehicle that has moved "flat out", or more than 12 inches in a turn. This is again from p70 of your rulebook.
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In general, a vehicle and the passengers are still two different units. So you must move one, THEN the other. All of the following are permissible:
* Vehicle moves 12", passengers all end their move within 2" of access points and embark.
* Vehicle moves 12", passengers already on board disembark within 2" of access points. They cannot move any further, and they cannot make an assault move.
* Passengers all end their move within 2" of access points and embark; vehicle has not moved yet so now moves up to 12 inches (cruising speed).
* Passengers disembark from a vehicle that has not yet moved; they disembark within 2" of access points and then make a regular move action (and later, can make any assault move they normally could). Vehicle now makes its move, and can move up to 12 inches (cruising speed)