Really, vampires are whatever you need them to be for the story. In most older folklore, they're more or less about fear of death, decay, and corruption. For Bram Stoker, they were basically fear of STDs that no one was acknowledging existed. In the modern times, they can be fear of stalkers, date rapists, abusive partners, authority figures, whatever.
First, ask yourself "what purpose to vampires serve in my story?" That'll give you a basic starting point for what "kind" of vampire you want to make, whether it's the modern sexy vampire, the slightly older disturbing vampire, the older-still monstrous vampire, the even-more-older rotting corpse vampire, or the oldest-of-the-old incorporeal vampire. That'll also help you figure out what weaknesses make sense. Are your vampires legitimately cursed by God? Then holy objects (or maybe just crosses) will work. Do humans have some special ability to resist them? Then holy objects will probably still work, but for different reasons. Are they literal "creatures of the night?" Then sunlight fries them into crispy critters. If not, maybe sunlight just weakens them, or has no effect at all. Are they abominations of nature, causing natural and physical laws to break down around them? Then you have a reason for them to cast no reflections and such.
The important part isn't necessarily finding definitive answers to why vampires are the way they are, but why they are the way they are in your story. There's so much vampire fiction and folklore that "accuracy" isn't so much a moving target as it is a nonexistent one. For instance, sunlight destroying vampires is wholly an invention of Hollywood, created for the film Nosferatu as one of the changes necessary to avoid a lawsuit for being an unlicensed adaptation of Dracula. Coffins are also a Hollywood invention, this time from the Dracula films, Bram Stoker originally just had "boxes of earth," since Dracula had to sleep in his native soil. So you really are free to use, modify, and discard any particular rules you want, and create your own if it serves the story better.
I hesitate to go into anything more specific, since I have my own series of vampire novels I'm writing (self-published the first one), and don't want to give away too much about how the vampires in that world work just yet.