Am I missing something?
I don't see anything in the Creature Guide etc. about undead such as vampires not liking sunlight. Leaving aside other lore, earlier edition warhammer had vampires not liking sunlight and some other things.
Did I miss a generic reference somewhere in rules or has 3rd edition dropped the sunlight problem for vampires (which drops quite a drawback for them).
If houseruling, I would have the Blood Pool tracker lose one space per minute in direct sunlight, and give the Vampire a Challenge die as if "blinded" (translating earlier edition). At "empty Blood Pool" it would face an Automatic Critical each round, with "dissolution" when the # exceeds To (to the extent a vampire is ever destroyed, once again using earlier edition concepts).
Some vampires might have the Defy the Dawn ability from Night's Dark Masters (call it a support action in this edition), an Easy Willpower check for one hour in the sun, next hour add a Challenge die and repeat. More likley found among Lahmia or Von Carstein types that like to socialize and less among Strigoi or Necrarchs who like the night for hunting and other reasons (mechanically take other actions instead). Strongly overcase days would let any vampire attempt this without the ability.
It would be easier to spot them for what they are (or at least for something unnatural) in sunlight (for those that have any chance of appearing human to start with), say a couple of fortune dice on the test.
Rob