Vapour pressure of solution

What will be the vapour pressure of the solution when non-volatile solute is added to a solvent?

A non-volatile solute in a volatile solvent decreases the vapour pressure of solvent. The reason is that molecules of solute which are non-volatile are also present on the surface of the solvent. So the number of molecules of the solvent per unit area of the surface becomes less and so the vapour pressure becomes less.

What is called the pressure under which liquid and vapour can co-exist at equilibrium?

When the vapours and liquid co-exist then there is equilibrium between them at a constant temperature. The vapour pressure at this stage is called normal vapour pressure of the liquid.