My tuppence,
More sleep and more veg gave me more energy, but I also took a fairly generic miltivitamin when I was working away from home, my diet was fine but working a full week, plus 15 hours in the car, plus maintaining some sort of social life and getting in some exercise was leaving me like a zombie. Multivit did seem to cure that, it wasn’t an immediate placebo effect, but after a couple of weeks I no longer got to Thursday with bags under my eyes unable to peel myself out of the car. May have been one particular vit/mineral that did it, but the mitli’s were cheap.
As for colds, zinc seems to work, and that’s actually scientifically proven, not just a placebo. IIRC VitC does also stave off colds, but you don’t need >100%, but it does need to be everyday, taking it whilst you’ve got the cold had no effect.