Vitamins give 10 EVs in the corresponding stat per use, but cap at 100 (or 10 uses).
EDIT: as of SwSh, Vitamins no longer have a cap.
Wings give 1 EV in the corresponding stat, but don’t have a cap.
In Gens IV-VI, power items give 4 additional EVs in the corresponding stat each time the holder gains EVs in battle. In Gen VII, they give 8 additional EVs.
Macho Brace doubles all EVs gained through battle. Does not affect vitamins or wings.
Pokérus doubles all EVs gained through battle. Stacks with Macho Brace and the Power items (multiplier applied after the Power items’ effect), but does not affect Vitamins or Wings.
EVs given by the Join Avenue Dojo varies