Hypervigilance is a symptom of hyperarousal. When someone develops PTSD (post traumatic stress disorder), one of the group of symptoms that develops is hyperarousal. After a traumatic event and when you have PTSD, your body has the fight or flight response triggered constantly, causing multiple arousal symptoms, where you are jumpy, easily startled, easily rattled, and always on high alert for any danger that still may be looming. This last symptom of hyperarousal, where you are constantly scanning your environment for danger, is the definition of hypervigilance.
So hypervigilance is a symptom of hyperarousal, which is one of a group of symptoms present in PTSD.