What is ideas about vapour pressure to explain how impurities increase the boiling point of water?

The boiling point is the point at which the vapor pressure of the liquid reaches atmospheric pressure. So, if you add impurities (solutes) to the pure liquid, the theory is that those molecules get in the way of the solvent molecules, and at the surface that results in a LOWER vapor pressure of the liquid. This then results in an INCREASE in the boiling point, because remember that boiling point is when vapor pressure = atmospheric pressure.