You could make it so that blood is simply a craving.
A lot of people crave sugar and fatty foods despite the fact that they offer little nutritional benefit and have lots of downsides, yet people still eat plenty of them and it makes them overweight and causes plenty of issues such as type 2 diabetes.
Sugar does actually provide energy in moderate quantities, and gorging on plenty at once can make one hyperactive. Having absolutely no sugar in a diet can equally cause problems, or mean that the person has little energy.
So substitute sugar or fats for blood and it's a similar scenario. Maybe lack of blood would make the vampire lack certain things, they may become some form of malnourished without blood, or are affected more by sunlight, but can still survive reasonably well.
If they moderate their blood intake then they can be completely healthy, but when they drink blood they will obviously crave more, which can soon cause them to get out of hand and they become addicted to it. Then an excess in drinking blood, rather than making them overweight would cause them to have less humanity and morality.
Maybe certain vampires have no blood in order to ensure their systems are not used to it, then they have parties where they gorge on blood and temporarily become incredibly energetic/strong/nigh-immortal.
There are people who exist almost solely on sugar and fatty foods, and whilst they are not particularly healthy they can continue to survive for a long time. So perhaps a vampire can exist solely on blood, but again it causes a number of issues.