A vow is a commitment to others. Driven characters want something for themselves. It may be to protect the realm, become a decorated officer, prove you’re the best gladiator in Rome, or the best pilot in the galactic fleet. The Minor version shapes the character and pushes his decisions but either happens rarely or is fairly harmless. As a Major Hindrance it’s an overriding desire that comes up frequently or causes peril for the heroine and those around her.