Sky Skiff

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.

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.