How do you become a vampire that can walk in daylight?

Sensitivity to sun-light in a vampire is dependent on two factors: the completeness with which the vampire was initially formed, and inheritance.

Becoming a vampire is actually a delicate process and if it is not conducted in the most appropriate manner the resulting vampire can be grossly mal-formed. Sensitivity to light being one of the factors that can be 'warped'. Certain vampires can improve upon their 'physical health' and through 'heavy feeding' and with abundant 'healing time' can 're-achieve' a light tolerant condition. Such healing however often requires a long time (decades at least) and a lot of nourishment.

Certain vampire blood-line have become victim to genetic mutation (a danger with all genetically driven things) and light sensitivity is one of their symptoms.