How can we use the concept:
Since the vertex is a point, it is named as all points are, with a single capital letter. For example, consider the following angle:

Point B would be the vertex. The name of this angle is ∠ABC. Angles are generally named with three letters, where the middle letter is the vertex and the two other letters are points that the rays, which form the angle, pass through.
A vertex is also a point where the sides of a polygon meet. The plural of vertex is vertices. Consider the following polygon:

This hexagon has 6 vertices - points A, B, C, D, E, and F.