Question

why is it not proper to regard the gesous state of ammonia as vapour

Answer 1

The word "vapor" refers to the gaseous form of a substance that is either a liquid or a solid at "normal" temperatures. Ammonia exists as a gas at normal temperature, so it is referred to as "gas", not as "vapor". 

                                                                     or

That is because, ammonia remains in gaseous state at room temperature, so it's called a gas.
We call something a vapour when that element remains a solid or a liquid at room temperature but vaporizes at a temp. higher than the room temp. but a gas is in gaseous state even at room temp.

Answer 2

The word "vapor" refers to the gaseous form of a substance that is either a liquid or a solid at "normal" temperatures. Ammonia exists as a gas at normal temperature, so it is referred to as "gas", not as "vapor". 

                                                                     or

That is because, ammonia remains in gaseous state at room temperature, so it's called a gas.

We call something a vapour when that element remains a solid or a liquid at room temperature but vaporizes at a temp. higher than the room temp. but a gas is in gaseous state even at room temp

hope its helpful