Like many things, it depends. Your calculations are quite reasonable and 372 should give your vampire enough, in fact it has a decent margin of error in it. It has a decent margin of error because a healthy person can give more than 1 BP in a night. I don't have the book handy, but I'm pretty sure they can give up to 3 BP before their life is endangered. Also a healthy person can give blood more often than the medical guidelines indicate. Further, the vampire can, at least to a degree, self regulate and avoid using unnecessary blood. You will still average well above 1BP a night, but a vampire that avoids fights and is deliberately conservative about blood use can probably come in under 2-3 BP a night on average.

So, as you say 372 will provide enough, and even give some safety margin. But that is 372 adults that are at least healthy enough to "contribute" three times a year. You are asking how many would be in a community to give you those 372 healthy adults.

And that depends. You want the birth rate to be equal to or greater than the death rate, but death rates are highly variable in different time periods and different parts of the world depending on circumstances. In the United States, for instances the crude death rate is 8 per 1,000 people per year. Going with that number and ignoring the fact that that is per 1,000 in the entire population and we are only looking at health adults in our 372, he can expect to lose around 3 of his herd each year. That means to be sustainable he needs 3 that are about to become old enough waiting in the wings. If we say they are "adult enough" at 16 and we want this to be indefinite we need 3 children at each age between newborn and 15 waiting around. So, in addition to our 372 adults, we need 48 children waiting around.

Incidentally, the crude birth rate in the US is 14 per 1000 people, so for our 372 adults we could expect an average birth rate (if comparable to US) of 5, easily more than the 3 we need.

Currently, this brings us to 420 to be sustainable. But, you also won't be able to count those with chronic infirmities that prevent them from contributing, like the very elderly. But how many elderly you have also depends. If the death rate is high, you probably won't have many. If you have customs of senicide (killing or suicide of the elders), then again you won't have many. If the vampire has a firm hold and wants to keep the population to a minimum, he might well create such a custom or else "cull" his herd of any not of use to him. If he does this ruthlessly, than 420 is enough with a decent safety margin. But, if he doesn't do this then he will have some additional people. In the US our population over 65 is 13% of total. With a base of 420 people, this makes 483 people total (13% of 483 is 63), so if this is a rough estimate of the number in his population he can't eat than we are up to 483.

I'm ignoring the temporarily sick because (unless there is an epidemic hitting a large percentage of the population) he can easily bypass them for a week and just come back. In short, they are absorbed into the safety margin.

So, I've played rather loose with a lot of the estimates and erred on the side of the vampire being cautious, but a population of 483 could sustain a vampire forever. This of course adds in the assumption that there won't be any huge deviations from average. If there is a war or plague or famine that wipes out a large portion of the population, he will have trouble.

Yes, and easily found

Go to the White Wolf Wikia, and check the respective category.

The Ventrue Chronicle I didn't read myself, but it seems so from the description that it's the thing you need, providing some plot for Dark Ages: Vampire.

You may get it at DiveThruRPG for 12$ as PDF -- you probably can't afford to wait for the delivery of the paper version if you only have 4 days! The advantage is that this book actually has 3 stories, one for the Dark Ages, one for Victorian Age, and one for the Final Nights.

Dark Ages: British Isles is another book, which may be bought here. However, it has all 4 lines of supernaturals together, so I hope you know them all if you gonna play it. If you get it as PDF, don't forget to download Dark Ages: British Isles Casualties to see the cut content with more details.

Also if you want to learn the setting, you better buy other books from the series than a book on history, because the former will get you more useful information for actual play.