Vitamins in the base layer
Vitamin C, the B-complex group, vitamin D, and vitamin E make up the familiar core of most men's vitamin formulas. B vitamins appear in energy metabolism discussions, which is why they show up in both daily multis and energy and endurance formulas. Vitamin D is included because dietary intake is often limited. Vitamin C and E appear in antioxidant contexts. On a panel you will usually see them listed with a percent-of-daily-value figure next to each one.
The reason the base layer stays stable across brands is simple: these are the vitamins with the longest history of use and the clearest reference figures. When a label lists a percent-of-daily-value, that figure is the fastest way to compare two formulas side by side, because it already accounts for the different units the individual vitamins are measured in. Reading the percent column first, before the milligram column, will tell you more in ten seconds than scanning the ingredient names.