Sunlight is one of the basic weaknesses of all vampires except for the The Old Ones. As time progressed over centuries, normal vampires are naturally weak against the sun.
Some vampire have charmed daylight rings with lapis lazuli; these rings must be forged by witches because the lapis itself is simply a stone. It protects them from the sunlight. So as long vampire wears it in the sun, he or she can't die from exposure. Isabella is in possession of a ring containing lapis lazuli which protects her from the harmful effects of the sun. However, without such a ring sunlight will burn a vampire on contact.
Under normal circumstances, daylight can be harmful to vampires. And depending on factors (i.e. the intensity of the light, length of exposure to it, along with how old and powerful the vampire in question). The effect may range from merely causing the vampire to sizzle or steam, to incinerating them instantly on the spot.
In Ancient Greek Myth, the sun was under the rule of two gods, the first being Apollo who was the God of Sunlight and under his rule, the sun was seen as a symbol of fire and destruction, under his rein, the sun burnt down hoses and killed the crop.
The second was Helios who was kind and fair and was the First Lord of the Sun. Under his rule, the sun was seen as a symbol of riches and gold, its sunlight gave life to the living and the crop and Helios drove the sun the sky in his chariot with his fiery horses which he named: Pyrois, Aeos, Aethon and Phlegon.