How can i change post processing at run time using if statements

basically im creating a bullet time game where time slows down if the player is aiming. I also want the vignette intensity to increase from the value it is to the higher value i want it to be and then when the player stops aiming to change from the higher back to the lower. I also want this effect to be smooth, so probably using Mathf.lerp. Ive followed a few tutorials on how to change postprocessing at run time and can do that but somehow when i use if statemtents it messes everything up.