In my setting, vampires are weak to the sun not because they burn up in it but because only in sunlight are they truly mortal. If they get buckshot through the chest and don't drink someone by 5am, they succumb to the hole in their chest. When they finally die, is when they turn to ash.
A vampire caught in the sun will have red steam erupt from their skin as all the stolen strength is confiscated by the light of prominence. Wounding the vampire expedites the process. Even if the sun doesn't immediately kill them, they still must avoid it as it'd be to easy of a tell.
To this end, a vampire just reborn will be easily killable after the first exposure to daylight, while an ancient vampire that has outlived several empires would be a morning long slog to finally bring it down. But as long as it doesn't retreat into shadow, it can and will be brought down.
A vampire feeds not for sustainable but to become more powerful and regenerate wounds. In terms of longevity there is no difference between a fully fed vampire and a vampire that hasnt fed in a millennia. They are reservoirs not engines: They exhaust blood when they need to.
Despite this, the vampire is constantly haunted by every single person they drank to death. The only way to silence the specters of their sins is to drink more blood, imparting a soporific effect that while not completely drowning out the screams, makes them far more quiet.