A bizarre neurological side effect of HVV's modifications to the sensory system and the brain gives every Vampire a neuropsychiatric disorder in the form of a psychosomatic hunger for human blood. The psychosomatic reaction to the smell and taste of human blood even triggers a dopamine response in Vampires. Neural pathways activated in Vampires during feeding are much like those found in addicts when using drugs and the dopamine released during this feeding on blood has a narcotic-like effect, including an addictive dependency.
By chance, HVV causes metabolic functions to be altered and enhanced to such an astonishing degree that vampires require far less nutrients than uninfected humans. As a result, even the minimal amount of iron, calcium, carbohydrates, fats, glucose, amino acids/proteins, and vitamins contained within human blood is actually enough to adequately meet the nutritional needs of a vampire. So a vampire can actually survive solely on a diet of human blood.
Because of the brain and sensory modifications, vampires find the smell and taste of ordinary food (e.g. animal meat and plants) to be foul and because of alterations to the digestive system caused by HVV, a vampire will become incapable of properly digesting solid food after a few weeks on a blood diet. The alterations to the digestive system also includes anticoagulants that happen to stop blood from curdling in the stomach. That happens to prevent any possible vomiting from ingesting blood, which is what happens when normal humans drink a large amount of blood.