Vow of Peace from Book of Exalted Deeds expressly forbids causing damage or killing anything other than undead or constructs. However, damage dealt to or killing of a summoned creature has no effect since all is reverted when the spell ends.
The context for this question has to do with a planned NPC character that the players will have to escort for a specific quest. I know that the DM can wave his hand and say NPCs can do whatever they want, but I prefer not to cheat for my non-player characters in my campaign.
Ultimately, I need to know two things: firstly what the NPC can do to assist players in its own defense, and secondly what the players can do to defend the NPC while preserving its vows. Prerequisites should also be preserved.
I realize that mention of some of the vows from this book, especially vow of peace, trigger some people. Please rest assured that I am not torturing my players. There is a specific, plot-driven quest for which this will be relevant.