A vigil is when you stay alert to guard something, as when you keep vigil over your hen house when the foxes are out. A vigil can also be solemn, as when a candlelight vigil is held for victims of a tragedy.
Vigil comes from the Latin word for "awake," and all its meanings include the idea of watchfulness. If you're attending a vigil or keeping vigil, you shouldn't expect to take a nap. The vigil is an apt kind of demonstration for peace movements, as it's calm, non-violent, and solemn, with participants standing quietly — sometimes holding candles.