Vehicles you control become artifact creatures until end of turn. Creatures you control get +2/+0 until end of turn.
The set of Vehicles and creatures affected by Start Your Engines is determined as the spell resolves. Creatures you begin to control later in the turn won't get +2/+0, and Vehicles you begin to control later in the turn won't automatically become creatures and won't get +2/+0.
Vehicles you control become artifact creatures and then also get +2/+0.