"Contrary to some beliefs, the Vampire, like any other night creature, can move about by day, though it is not his natural time, and his powers are weak."
-Abraham Van Helsing (Anthony Hopkins), Bram Stoker's Dracula (1992)
The most popular notion in modern Vampire folklore is that the Vampire cannot withstand direct exposure to the rays of the sun. Most often, the creatures react very violently to it, often bursting into flame or even exploding! It should be emphasized that the Vampire of folklore had, with only a handful of exceptions, no such vulnerability. In fact, one particular Vampire from Russia (the Upir) is known to hunt from noon till midnight, and returns to it's grave afterwards. It would seem that the only species of Vampire that exhibits such a vulnerability is the Jiangshi or "hopping vampire" of Chinese lore and legend (which is one of the few things that can kill the creature). In Bram Stoker's 1897 novel Dracula, even the Count himself could walk about in direct sunlight without ill effect (however, his powers were greatly weakened during the day). Yet he was still powerful enough to take down a careless Vampire Hunter.
This particular misconception came into being with the first showing of F.W. Murnau's 1922 silent horror masterpiece Nosferatu: Eine Symphonie des Grauens (A Symphony of Horror). It portrays actor Max Schreck as Count Graf Orlok, and in the ending, Orlok is shown, upon exposure to daylight, to fade away, leaving nothing but a puff of smoke. These days, death by exposure to the sun is a popular device in film and literature, useful only for destroying or limiting the Vampire's activities. This was the first sign that people had forgotten the horrifying truth of the old legends...
In 2008, author Stephanie Meyer created the Twilight series, chronicling the adventures of the Vampire Edward Cullen and his human love interest, Bella Swan. What makes this series relevant to this discussion? The fact that Edward and his family's skin, upon exposure to direct sunlight, begins to sparkle like thousands of diamonds. To think that something like this has entered into and been widely accepted by Vampire fans is not only ludicrous, but it is also one of the queerest things that this author has ever seen!! Where this notion came from is unknown, but it appears to have gained popularity with teenage girls and grown women alike. Both seem to want a dark, handsome man who is not opposed to such an open display of femininity. Moving on...
Nowadays, destruction by sunlight is considered to be gospel when it comes to Vampires. Very few are even remotely aware that this is completely fictional, and that real Vampires are not so easily dispatched. With some exceptions, the Vampire of olden times is able to come and go as it pleases from it's grave, regardless of the time of day. This is one of the things that made the bloodsucking undead so terrifying. An attack could come at any time of the day (although, in most cases, the Vampire prefers to rest in it's grave during the day). Because the Vampire is primarily a nocturnal creature, people have just generally assumed that sunlight must have some adverse effect on the creature.
Another reason that the Vampire hunts at night is because people sleep during the night. They are vulnerable at this time, and far less likely to resist the Vampire's depredations. The Vampire may be a reanimated corpse, but it is not without intelligence or cunning. The Vampire knows that patience is the key to a successful hunt, and in the long run, patience usually pays off.