Aradara Express

When a Vehicle becomes a creature, that doesn't count as having a creature enter the battlefield. The permanent was already on the battlefield; it only changed its types. Abilities that trigger whenever a creature enters the battlefield won't trigger.

Each Vehicle is printed with a power and toughness, but it's not a creature. If it becomes a creature (most likely through its crew ability), it will have that power and toughness.

Vehicle is an artifact type, not a creature type. A Vehicle that's crewed won't normally have any creature type.

You may activate a crew ability of a Vehicle even if it's already an artifact creature. Doing so has no effect on the Vehicle. It doesn't change its power and toughness.

If a permanent becomes a copy of a Vehicle, the copy won't be a creature, even if the Vehicle it's copying has become an artifact creature.

Once a Vehicle becomes a creature, it behaves exactly like any other artifact creature. It can't attack unless you've controlled it continuously since your turn began, it can block if it's untapped, it can be tapped to pay a Vehicle's crew cost, and so on.

If an effect causes a Vehicle to become an artifact creature with a specified power and toughness, that effect overwrites the Vehicle's printed power and toughness.

Any untapped creature you control can be tapped to pay a crew cost, even one that just came under your control.

Once a player announces that they are activating a crew ability, no player may take other actions until the ability has been paid for. Notably, players can't try to stop the ability by changing a creature's power or by removing or tapping a creature.