Addition of HgI2 to KI (aq.) shows decrease in vapour pressure. Why?

HgI₂ forms a complex with KI and therefore number of particles in the solution decreases.
HgI₂ + 2KI $\to$ K₂[HgI₄] 
Vapour pressure is a colligative property, decrease in number of solute particles will increase the vapour pressure. Therefore it will show increase in vapour pressure not decrease.