A ray diagram in the context of mirrors is used to describe the path taken by the incident ray of light and that taken by the ray of light after it has reflected off the mirror.
For any mirror, the angle made by the incident ray when it strikes the mirror with the normal drawn at that point is the same as the angle made by the reflected ray with the normal.
As you have to draw a ray diagram for a vertical mirror, the incident rays make an angle of 0 degrees with the normal. The reflected rays too makes the same angle of 0 degrees. This indicates that the incident ray is reflected back along the same path it came from. The only difference between the two rays is in their direction.