Antifreeze/Coolant is a solute that is mixed with water and placed in the radiator of cars. It doesn't affect the vapor pressure of water at all. The addition of a solute to water lowers the freezing point and raises the boiling point. This is important for the function of the engine, because internal combustion generates a lot of heat energy as a byproduct of converting the potential energy in the fuel to kinetic energy. This heat could very swiftly melt some engine components if not dissipated. The water/coolant solution absorbs the heat as it flows past parts of the engine in metal pipes (metal conducts heat), it then flows through the radiator baffles where the heat is conducted to air flowing through the grill of the car.
In hot weather the solute keeps the water from boiling into a gas even at temperatures in excess of 212F. In cold weather the solute keeps the water liquid between drives, even at temperatures less than 32F.