This is a great question that strikes at the heart of much of European folklore. Many entities are more or less associated with death and yet have seductive qualities. It's a strange tension - isn't it? - that allows for death to be both horrifying and alluring.

We see this with the ever-so-seductive mermaid who calls on sailors to fall in love with them when their only real goal is to drag them down to water graves. We see that in the famous "Lenore" poem of the late eighteenth century, which is based on the legend of Spectral Bridegroom (Tale Type ATU 365; see my article on the subject ). Here we have a bridegroom killed in battle, returning as a walking (or riding) corpse, calling on his bride to come with him to his grave, and she yields, seduced, only to escape at the last minute - but she usually dies of fright when she fully understands what really happened.

Even the elves/fairies/hiddenfolk of Northern Europe can become extremely seductive, and yet to yield to them and enter their realm is to find oneself in a place that is very close to death. In fact, these supernatural beings are often thought to waiver between death beings and entities of the forest or the field.

And here he find an answer to one of your questions, namely about a vampire's immortality: they are not immortal, but rather they belong to a realm that is eternal in the same way that death is forever. These entities do not live forever. They exist forever, and humans who are brought into their domain are deprived of the salvation of death in that they cannot complete the life cycle and enjoy heavenly redemption. Rather, they are cursed to belong to that eternal death-like existence of these supernatural beings.

Like many of the more horrifying creatures of European folklore, the vampire exhibits the most dreadful aspects of death and yet is able to be seductive at the same time. And like many of these other creatures, there is a tendency to be shy of daylight. This is partly a matter of people being able to see more clearly during the day: we don't see the monsters we dread during daytime, because we can see things, but at night, our vision is limited, and it is easy to imagine anything lurking in the deep shadows. This causes much of folk belief to conclude that these creatures cannot endure the sunlight. It is a logical extrapolation: these creatures exist, and yet we never see them during the daylight, and we only get glimpses of them at night (or so we imagine); therefore, these creatures can only be afoot during the night; and therefore, they abhor or cannot endure the sun.

Others can address the literary evolution of the seductive vampire, but the answer from folklore is that there are many creatures lingering on the edge of death who are equally seductive.