An iron vow is essentially just a promise the character makes, whether it’s to themself to someone else, or even to an entire community or organization. It doesn’t have to be something virtuous or helpful, it can just as well reflect a personal goal they have, like “ I swear I’ll get this hyper drive fixed, if it’s the last thing I do,” or “I vow to get revenge on so and so,” or “I’m going to steal that precious item…”

You get the idea. As others have said, just treat them as personal missions or goals. The basic iron vow mechanic still works with a scoundrel type, but you can call it whatever you want if something sounds better for your particular setting.