Why is the setting important to the themes? It was taken place in England and Eastern Europe during the end of the nineteenth century.
If you go to the link below here on eNotes, you will find some helpful information on the historical context of Dracula that will answer your question. The values that were prevalent during Victorian England have a huge impact on this story because just before it was written, Jack the Ripper was terrorizing Londoners. Also, Victorians were very concerned with disease because at this time, London was a cesspool of disease and medicine was quite primitive. Doctors were still bleeding people in some cases. So, all of this adds to the gothic nature of the story. Also, most Londoners had not traveled much and Eastern Europe was considered somewhat exotic, a country of dark mountains, deep mysterious forests, and strange counts that lived in Transyvania such as Count Dracula. The mystery of the unknown enhances the mood of the story. In well-known, familiar, rational, scientific England everything was explainable, but in far-off, unfamiliar, irrational, spiritual Eastern Europe -- ahh, who knows what might happen?
The gothic setting in Eastern Europe enforces the theme of salvation vs damnation because vampires are the un-dead. They cannot find eternal rest because they must wander the earth getting more blood to sustain their state of existence, which is between life and death, and therefore damnation. Also, the vampire motif relates to the theme of science vs the supernatural. Even though Jonathan believes that what he is experiencing cannot be true because it is scientific, nevertheless he finally realizes that he is not mad and he has indeed experienced vampirism, so perhaps he is wrong about the nature of reality? Van Helsing certainly believes that science just cannot explain it all.