Luck and his crew were trying to stop the Demon Lord or w/e but, what they didn't realize back then was that the Demon Lord was just trying to summon some Evil God.
The Vampires Luck deals with after his return is the "Plan B" to summon the Evil Gods. so the whole story is basically about Luck hunting Vampires to stop them from completely Summoning the Evil God. the story is Luck being kinda OP and stopping the Evil God from being summoned, without the vampires there is no threat of the Evil God being summoned, without the threat of the Evil God being summoned there is no story