The Vengeance Sworn should have been called "The Riverwind," as the class is pretty clearly tailor-made for that one pre-generated PC from the original Dragonlance campaign. The requirements are just stupid, even apart from the "death (of self or loved one) by dragon" aspect; most of them have no obvious connection to either the powers or the role-playing function of the class:
Skills: Intimidate 5 ranks, Knowledge (arcana) 5 ranks, Survival 5 ranks
Feats: Ambidexterity, Two-Weapon Fighting, Combat Expertise
Base Attack Bonus: +6
Spellcasting: Ability to cast 1st-level divine spells.
Special: Rage class feature.
Special: Must have been killed by a dragon or had a loved one killed by a dragon.
It works for Riverwind because, aside from having his whole village and its people burned by the red dragon Ember, and being slain himself by the black dragon Onyx (then raised with the True Resurrection capability of the divine magical artifact his lover, Goldmoon, was carrying around at the time), he just happens to begin the campaign (in the 3rd Edition version, at least) as a Barbarian 3/Ranger 2 with the two-weapon ranger combat style and five ranks each in Survival* and Intimidate. All he has to do to qualify is take Combat Expertise and put all his skill points into Knowledge (arcana) when he gains his sixth character level.
*Have you ever heard of a ranger or barbarian who didn't max out Survival? Aside from Belkar Bitterleaf, anyway -- with his Intelligence penalty, the little psychopath probably only gets one or two skill points a level, and he presumably puts most of them into Intimidate, and then Hide or Move Silently, the better to sneak up on people and murder them.