I'm writing an urban fantasy story revolving around, among other things, vampires secretly living in the United States in 1998. And since the first two vampires came into existence in the area that is now known as Romania in around 1450, a question I need to have a solid answer for is when, exactly, they came to America, and more to the point, when that would have been physically possible.
Sea travel is a bit of an issue for vampires, you see, for two very important reasons:
1: While the strongest of them can do things like fly, manipulate the weather and control human minds, they have a weakness to both the sun and open water. The celestial energies Earth radiates out from its core take on different properties depending on what non-gaseous material they last passed through. If it was more than three feet of water followed by less than three feet of any other solid matter, it will be as if a vampire were standing in direct sunlight. Either sunlight or open water on their own would rob a vampire of all of their advanced powers. Exposure to both at once would further rob them of their strength, leaving them weaker than the average human (and those are the most powerful; to any lesser vampire a combination of sun and open water would be utterly lethal). So no flying across the ocean; they need a ship.
2: In my setting, magical beings such as vampires cannot under any circumstances stray from human civilization for longer than a month, specifically during the full moon. A phenomenon known as moontime occurs every full moon, and it keeps the supernatural population in check by making any area that's seen excessive immortal foot traffic (more than 1 in 1,000 humans on average over the course of the month) exceedingly lethal for the immortals caught there during the full moon. Being caught out at sea with nobody nearby but the humans on a single ship, therefore, would be a death sentence. So the journey across the sea has to take no more than 30 days.
I don't expect provisions to be an issue for the voyage. Vampires require a pint of blood every 12 hours they are awake, but can hibernate during daytime and they possess a magical method of preserving human blood in glass bottles, so no need to start draining their crew.
With these facts in mind, when is the soonest that human seafaring technology could have allowed a vampire to survive the voyage from Europe to North America?