"Vengeance is walking in Salem!" means that people are using the witchcraft accusations to get revenge on those who they believe have hurt them, or those against whom they feel jealous, or angry, or just negative in general.
In The Crucible: Act II, the accusers are carrying out their own personal grudges in the courtroom.
Inspired by the McCarthy hearings of the 1950s, Arthur Miller's play, The Crucible, focuses on the inconsistencies of the Salem witch trials and the extreme behavior that can result from dark desires and hidden agendas.
For more about The Crucible, see CliffNotes.
Check out these 10 Witchy Facts About 'The Crucible' compiled by Mental Floss.