The history of vampires in works of fiction and folklore goes back a very long time and they've appeared in stories and legends from many different cultures. Anybody with the inclination to delve into the variations on their origin that different cultures have devised would be in for a real treat as there are dozens of them. In some cultures, people might rise from their grave to become a vampire if their grave were to be disturbed after they were interred. People in other cultures believed that people morphed into vampires if their bodies hadn't been properly cleaned and dressed prior to burial. In more modern times there have been a host of different methods of creating new vampires ranging from curses to being bitten by an existing vampire, and many authors have simply worked it into their books and stories that for all intents and purposes, vampires have always existed, and the split between humans and vampires happened so far in the past that the story of their origin has been lost in the mists of time.
So it would seem that you have a great many options available to you if you would only look into them. It's rather strange that you find the few you've considered already to be too unbelievable. I mean, it's not as though you've met any actual vampires as to the best of our knowledge, they don't exist. So what would make one method of creating one any more plausible than the next?
And if you intend to write a story, don't you think that you ought to summon all of the creativity and imagination that you can bring to the table to do it? Asking strangers on the internet doesn't count as doing real research. What you're doing is asking other people to do your footwork and your thinking for you. If you don't have the determination, the dedication and the diligence to work things out for yourself, then you probably don't have what it takes to write a story anyway. And before you foolishly begin to sling barbs at people who point that very simple truth out to you, take a minute to digest the inherent logic there.