Irontread Crusher

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.

For a Vehicle to be able to attack, it must be a creature as the declare attackers step begins, so the latest you can activate its crew ability to attack with it is during the beginning of combat step. For a Vehicle to be able to block, it must be a creature as the declare blockers step begins, so the latest you can activate its crew ability to block with it is during the declare attackers step. In either case, players may take actions after the crew ability resolves but before the Vehicle has been declared as an attacking or blocking creature.

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.