“Help” is a word that the vocal coach Eric Vetro uses often to describe his contributions to the careers of celebrities. This season alone, he’s helped several actors with the musical demands of roles as varied as Maria Callas (Angelina Jolie in “Maria”), Bob Dylan (Timothée Chalamet in “A Complete Unknown”) and a young Good Witch of the West (Ariana Grande in “Wicked”). He also worked with Monica Barbaro to honor Joan Baez’s vibrato in “A Complete Unknown” and Nicholas Galitzine to become a boy-band dreamboat in “The Idea of You.”