Vitamin C does boost your immune system and it should be a part of your normal daily diet. As to adding it to your diet, please speak with your doctor. One know danger is the grapefruit which can cancel some meds while changing the potency of others.
That said, when I feel a bad cold coming, I will supplement my diet with 1000 mg of Vitamin C each day until the bad feeling has passed. Afterward I just rely on my daily diet for my C through fruits and veggies. It has been several years since I have had a cold. Most of my problems have been MS.
Lastly, since surviving a near fatal strain of bacterial pneumonia in 1995, I have been taking an annual flu shot. That is my only annual shot other than my tetanus booster when needed. I plan to get the shingles vaccine the next time I go to my health department.