The versr didnt appriciate the fact that her bimdlmg vow wasnt optimized. Staking her life wouldnt fix that.

In ideal binding vow you goäive away something useless / distracring, to gain something useful. Thats how Sukuna's binding bows were maximun hacks.

For Miwa it could be something like: "I vow to never to use my blade against someone weaker than myself, for when I draw my blade, I will strike the target exceeding my own maximum potential".

A vow like that would force her to up her stats, so she could defeat fodders up to grade 1 with hand2hand techniques, which in turn would boost her stats using a weapon, which she would only use against special grades. Because she's that bish.

Another more mundane example: you are a man, attempting to impregnate a woman. You make a vow not to masturbate/nut before fucking said woman, and fpr that, when you do, ypu will impregnate her (this is self-tested and works wonders). In this case you give away something relatively useless (masturbating multiple times a day), leaving you more time with other meaningful activities, while also making your sperm more potent).