What better way is there to show the dangers of foreign influence then with metaphors and analogies using foreign monsters?
Vampires came to Japan in much the same way that Christianity did, on a boat from Europe. While Japanese mythology had creatures that had vampiric qualities, genuine, dyed in the wool vampires had to be imported. In Issue 293 of the The Escapist, Fintan Monaghan discusses how in the 1930's, when Japanese xenophobia was at its peak, the imported vampires became the perfect symbols to highlight the dangers of foreign encroachment into Japan.
[The] atmosphere of suspicion and paranoia [in Japan] was stoked by the most popular book of the 30s boom, Seishi Yokomizo's Dokuro-Kengyo (The Death's-Head Stranger).
The tale transplants elements of the Dracula story to Tokugawa era Japan where a mysterious stranger, Shiranui, transforms the Shogun's daughter into a vampire as part of a plot to bring down the government. The character of Shiranui was based on Amakusa Shiro, an actual historical figure who led a Christian rebellion in 17th Century Nagasaki. At that time, the shogunate worried about the creeping influence Christian missionaries were having on the masses, eventually leading to the religion being completely outlawed and access to the country closed to outsiders.
In the book, Amakusa is now a vampire, his deep immersion in Western culture having turned him into a kind of monster capable of spreading his corrupting influence to others. The vampire thus came to serve a function impossible for the homegrown Kappa or the Black Cat, embodying a generation's fears of seditious foreign influence and the dangers of western ideas.
While they're not used as a vehicle for turning foreigners into bogeymen anymore, vampires are still heavily tied into the idea of being somehow intruders and interlopers from other places. You can read more about it in Monaghan's article, "Vampires of the Rising Sun."