According to Raoult's law, the relative lowering of vapour pressure is directly proportional to the mole fraction of the solute. Adding more solute (like $\text{NaCl}$, $\text{Na}_2\text{SO}_4$, or $\text{KI}$) increases the number of solute particles, which further lowers the vapour pressure. On the other hand, adding water (solvent) dilutes the solution, decreases the mole fraction of the solute, and consequently increases the vapour pressure of the solution.