Good question. I'm used to reading about diets of blood. But since these vamps are genetically related to humans, the nutritional needs are probably similar.

For a large group gathering, perhaps the main course is a series of raw meats (beef, pork, lamb, goat, horse, etc, rather than much fish or poultry). Serve communally - I'm visualizing traditional Inuit feasts I've been to where the various cuts of raw caribou, muskox, char, and so forth are laid out on long sheets of waxed paper on the floor, and the folks (starting with the elders) just sit down next to whatever they intend to eat.

As for drinks, how about a broth or puree that includes vegetable and fruit contents, so they can avoid the hassle of eating their greens yet still get the nutrients. Perhaps the mode of service somehow simulates the blood-drinking experience?