When UV from the Sun hits a person, often there's a bit of damage to the DNA. There's a type of enzyme in our body that goes around fixing the damaged DNA. A person with Vampire Syndrome doesn't have the enzyme working properly - so they suffer continuous damage to the skin from the Sun's UV.
As a result, a person unfortunate enough to have such a condition has to be completely shielded from the Sun, or else they'll get extreme sunburn and skin will break down. If care isn't taken, they could also easily develop skin cancer.
There's no known cure for it and the best way to take care is to simply stay out of the Sun.