Vigilance

Granting vigilance through an Aura is the kind of single-keyword enchantment that lives and dies by the math: it costs a full card and a turn to do what creatures increasingly come stapled with, and the keyword it grants is the least disruptive one to lose your enchantment to. There is no body here, no card-advantage cushion, just a permanent that converts an attacker into an attacker that can still block. The structural problem with single-keyword Auras has always been the same: spend a card to improve a creature, and any removal spell pointed at that creature two-for-ones you. Vigilance has no rider to soften the blow, no toughness boost or evasion to make the investment harder to punish, which is why the effect migrated almost entirely onto creatures' printed text and onto equipment that survives the creature it suited up. As a piece of design it documents an early-era instinct: that a keyword was worth a card on its own. The years since have decided otherwise, and the keyword now reads as a line on a stat block rather than something you cast.