What does the stranger in the crowd vow in the scarlett letter?

Answer 1

The stranger is Roger Chillingworth, Hester's husband. He vowed that he will find the father of Hester's baby, which is Dimmesdale.

Answer 2

The stranger in the crowd, later revealed to be Roger Chillingworth, vows revenge on the father of Hester Prynne's child, whom he believes to be responsible for Hester's shame. He makes it his mission to uncover the identity of the father and seek retribution.