We generally have an image in our minds of certain historical periods. This image is coherent, consistent, and harmonious. That is, the technology, culture, and social life are on a corresponding developmental level.
The problem, when you mix what you call cultures is twofold. First, the image you create clashes with the image that we have of a historic situation. And this image that we have is very strong, because we have learned it in school and countless movies, books, and documentaries have reaffirmed it and added more detail. If you introduce something into a historic age that did not originally belong there, it will feel wrong. If you handle the contradiction well, you may achieve wonderful humor, as in the Flintstones example given by Alexander in his answer.
Second, historical situations are instable. History is a process of constant change. For example, there were no guns in the Middle Ages. When guns appeared in Europe, that meant the end not only of the knight, his armor, and medieval warfare, but also as a consequence the end of medieval courtly culture. When you introduce certain aspects of one culture into another, that culture will disappear. We all know what Christian missionaries did to the now-extinct cultures of the world. If you handle this destructive potential well and make it a theme in your narrative thought experiment, as Mark Twain did when he introduced modern technology to the Middle Ages in A Connecticut Yankee in King Arthur's Court, you may achieve an enlightening deconstruction of cultural stereotypes.
The great danger in a naive combination of cultures is an unintended and unvoluntary humor, or in other words, that some aspects of your story will appear to the reader as laughably implausible.
These are general thoughts, meant to be useful beyond the narrow scope of your own question. Whether or not Romanian vampires and nineteenth century Italy, or Italy and Roman government structure clash, you will have to answer yourself.
Personally, I have a very strong sense of the setting of certain elements of folklore and do not read books that "abuse" these cultural origins. Other readers will certainly be more open to fairies in spaceships.