Wouldn't flyting be a Thing, in that instance? And the grandiose speeches that heroes and villains give, monologuing as a free action (as we say in my D&D group). Those would literally be displays of power, as well as reckless overconfidence if you're vowing all over the place in public that your enemies will fall (and that their mothers dress them funny) etc. I figure for every time one cannot complete an oath (or deliberately fails to, or breaks one), there's a loss of some of your magical energy, up to and including life force, as well as painful backlash. That's if you're not so constrained by your vows that you cannot consider deliberately disobeying in the first place.
I keep thinking of thing that require a lot more brain power than I can bring to bear at the moment, but which I want to see. Edit- to clarify, this suddenly requires a much better head for poetry and literary references than I have.