Vapour pressure is neither bulk nor a surface phenomenon but, evaporation is a surface phenomenon. By the addition of non-volatile solute, the upper surface area occupied by solvent decreases due to which it evaporates less and will have comparatively less mole fraction than in pure solvent, because of this its vapour pressure decreases.
See the figure, when A+B is present A evaporates less and VP dec.
![](https://s3mn.mnimgs.com/img/shared/content_ck_images/ck_5e96928cc9dd5.jpeg)