Vapour is a name given to the gaseous state obtained by evaporation or vaporisation of the substance which exist as liquid or solid at room temperature. The substance which exists as gas at room temperature is called gas.
A more authentic difference between gas and vapours can be done on the basis critical temperature (T c) of the substance. It is a temperature above which a gas cannot be liquified at all by compressing it.
Gaseous substance above its critical temperature is called gas. On the other hand, gaseous substance below its critical temperature is called vapour.
For example, critical temperatures of H2, He, etc., are much lower than room temperature. Hence they are permanent gases while critical temperature of water is much higher than room temperature. Hence, water molecules in gaseous state are called vapour at room temperature.