If you’ve reached the vertex of something, you know it’s all downhill after that, because vertex refers to the highest point on an object, such as the top of a mountain.
In its early usage vertex referred to the top of a person’s head. That meaning is still common, especially in the field of anatomy, but these days you can use the word in a broader sense to refer to the highest point of anything. Still, it’ll help you remember the meaning of vertex if you visualize the top of someone’s head — the highest point — when you think of this word. If you want to get technical, in the field of mathematics, vertex can refer to the point at which lines meet and form an angle, or to the point on a figure, such as a triangle or cone, that is opposite to its base.