Well Heart isn't just charisma - it's also things like moral courage, determination, integrity, soul, etc. An iron vow is a sacred and spiritual matter so for me Heart is exactly right. You're not brokering a deal - that would imply negotiation and bargaining, but rather you're pledging your life to the service of another's needs.
But there is Fortune Hunter, so your concept could be another interesting twist on the premise.
How about...
1 - When asked to Swear an Iron Vow, you may request a promise of action from the recipient as a return favour should you be successful. If you do, roll +Wits instead of +Heart gain +1 Momentum on a hit.
2 -