When driving straight down the highway at a constant velocity you have to give the engine a little gas (which means an added external force of tires pushing on the road) to maintain your uniform motion. Explain why this does not violate Newton's 1st law. What are the other forces acting on the vehicle?"

This does not violate Newton's 1st law because the net force would still be 0 in order to produce uniform motion (aka constant velocity). The other forces acting on the vehicles is air resistance which is non-zero. So we need car internal force to counter balance this force, which require extra gas for the car.