I take the vitamins to shore up any diet deficiencies. VitD because we ain't get no sun here.
Whey protein I use occasionally to reach my protein goal.
Don't really notice anything directly. I suppose noticable things are kind of inferred. Like I build muscle so I infer that my adequate protein intake from whey combined with lifting is beneficial. And my doctor reports I'm very healthy so I infer that my diet and supplementing with vitamins is beneficial.