Maybe vampirism is caused by a retrovirus, but a rather intense retrovirus in that it seems to coordinate different tissue level effects.

Muscles and nerves become better coordinated. Muscles are more dense and there are more actin filaments. The receptors of the eye multiply making low light vision possible and the light of day unbearably painful. Healing is extremely rapid thanks to mutated fibroblasts that are better at coordination, and seem to have a memory of sorts.

Up until that point the vampire seems merely like a super-human. But there are more intense changes still that occur as the vampire ages. First, a new nervous network forms in the body -- the cardiac nerves. Cardiac muscle is spread throughout the body. Large veins and arteries are reabsorbed and a network of smaller, more uniform arteries and veins surrounded by thin bands layers of cardiac muscle spread throughout the body. These create diastolic waves that push blood throughout the body. The heart becomes a second nerve center and helps coordinate this new system to get blood across the body (it'd be a shame if some sort of resistor was jammed in it -- it would disrupt, but not stop, blood flow). There are smaller nerve centers spread throughout the body. These increase reaction times and act as a secondary torpor body maintenance system if the brain must shut down or go into 'low energy mode' for whatever reason. Bone marrow is largely shifted to the task of creating a superior immune system and super-fibroplasts. The old digestive and waste system is gone. It is replaced by a retro viral cell transformation factory of sorts.

There are drawbacks. The mucus membranes are very sensitive to pure water. It is corrosive and causes a severe allergic reaction. Also, the skin is very sensitive to higher energy light. It seems to trigger a 'hibernation' effect on the retrovirus in which the original infection form is reconstituted and it releases whatever water is in the cell and causes apoptosis. This is reversible if caught early enough or if the exposure is mild enough but seems to cascade if the exposure lasts for more than a few seconds. In other words, there is a critical mass that once crossed is irreversible and nearly complete.

Now we come to bloodlust. A vampire can create it's own blood cells but the facility is limited. They need to feed for three reasons.

First, energy. Their bodies need energy. They don't seem to feed enough to account for all of the energy they need which has lead some to some interesting theories on some sort of endothermic metabolism process but it is still a mystery.

Second: blood. They need human blood for their 'transformation factory' digestive system to repurpose for their own bodily function.

Third: Water. While it is possible for a vampire to consume water if it is mixed with other substances, it can be unpleasant.

Finally: waste removal. Vampires don't urinate or defecate. Their metabolic process seems very efficient but it still produces waste. They seem to be able to secrete a limited amount of waste directly through their skin and breath but, eventually, they need to release waste through their hollow teeth. Different human hormones seem to activate different waste glands -- cortisol and adrenaline seems to be needed most frequently and luteinizing hormone (released during sex) less frequently but just as vitally. Vampires also seem to be able to release chemical signals (perhaps the same way they release waste products) that trigger hormone release in humans.

Whelp, that's the best I could do off the top of my head. There is a lot of hand waving there but hopefully the weaker points can be ironed out.