Eternal Struggles of Immortal Demons
A major reason why Vepar had pledged allegiance to Eren and his Elder Ichor faction was because she had witnessed the level of authority he wielded over demons of her own caliber.
And this was when he was merely a Master Ranker.
Through firsthand experience, Vepar had discerned that Eren would inevitably ascend to become an immensely powerful entity, surpassing even the most venerable Immortals.
So, when he offered her the choice to serve him, she wholeheartedly embraced the opportunity to support his faction.
Additionally, Vepar's decision to align with Eren was fueled by a sense of betrayal from the Demon Prince of Wrath, Samael.
She believed that only someone like Eren could eventually pose a credible threat to Samael's authority, and supporting Eren's cause became her way of exacting retribution for the injustice she felt she had endured at Samael's hands.