Dragonfire adept player wants to gain Vow of Peace feat, which demands you to deal nonlethal damage to humanoids and monstrous humanoids. Is it possible or not? Please note, that he has Vow of Poverty feat, so no magic items.
Dragonfire Adept's breath effects do provide some options to deal status effects instead of damage (Sickening Breath, Slow Breath, Weakening Breath, Sleep Breath, Paralyzing Breath), which is compatible with Vow of Peace. But there isn't a breath effect nor feat to deal nonlethal damage.
It would be reasonable to house-rule a metabreath feat based on the Merciful Spell metamagic. The metabreath feats that do exist (Maximize Breath, Quicken Breath, etc) are payed for by increasing your breath weapon's recharge time by a number of rounds equal to the corresponding metamagic's spell-slot adjustment, and Merciful Spell has an adjustment of 0, so a hypothetical Merciful Breath feat would have no additional cost.