Transitioning from a mortal diet to a vampire diet...

I think it actually wouldn't be as hard to adjust to a vampire diet as many might think. If you really like garlic bread - then, yeah, you would have to give that up. Although, it might be possible to combine other spices to make imitation garlic bread.

Many bakery recipes use eggs - and, as it turns out, blood could be used as an egg substitute. So, right there, it provides a lot of dietary options for vampires. I figured the blood-as-egg-subsitute could also sorta work the other way around for vampires, to some extent. Egg alone cannot be used as a blood substitute for vampires - but, if you combine eggs with blood, you could extend the blood supply a bit. If you use a pint of blood, for instance - and then add the equivalent amount of eggs - then you would end up with about one and a half pint of blood nutriuents.

Tomatos and red chili peppers can also be good blood enhancers for vampires. To make marinara sauce for vampires, you could replace about half the tomato sauce with blood - then throw in some additional fresh tomatoes, and then use some extra seasonings (omitting garlic, of course).

Rare steak is always an option, as well - or even bleu steak, at steakhouses that are permitted to serve their steak that way. Various cultures have certain traditional recipes: such as blood sausage, blood pancakes, duck blood soup, or sanguinaccio dolce (blood-chocolate pudding or ice cream).

Mortals who grew up routinely eating blood-based foods would probably have the easiest time adjusting, although they would probably still notice differences to their palate.