ANSWER:

If they made the vow by taking Alhusain’s name then its shirk and its not a valid vow, hence there’s no expiation.

They must sincerely repent to Allah while showing deep remorse for this shirk and renew their shahadah

QUESTION: I Know That Making Dua To Death Person Is Shirk But If A Person Say That Ya Hussein If This Happens I Will Do This Here It’s Called Mannat Which Is What People Do So If I Tell Someone It’s Wrong And They Believe Me Will They Have To Do That Mannat Or Give Kaffarah For It if they make waseela to a dead person to pray to Allah