A vow is exactly what it sounds like. It is a promise that the client must make in order to move forward, but it is taken while witnessed by a higher power.
Depending on the situation, you will need to work with your client to come up with the appropriate promise. It is not always an easy promise to make, but you can ask your pendulum to confirm when they have chosen the right phrase.
While they say the phrase, allow your pendulum to swing clockwise as the Divine is present to witness it.
While less common than making a vow, sometimes you will find that your client is holding onto a promise that is no longer serving them. Ask your pendulum if it is time to let go of the promise and allow your pendulum to swing counter-clockwise as the Divine dissolves the energy of the vow.