Vampires in World War 2 and the fiction disclaimer

 I recently came across a book called Fiends of the Rising Sun, which features vampires fighting together with the Japanese against the Americans during World War 2.
The copyright page of the book features the following section:

Historical Note:
This novel is a work of fiction set during the Second World War.  As far as possible, the historical details are accurate but the story takes liberties with reality for narrative effect.

It doesn't spell it out, but I'm guessing some of those liberties have to do with the vampires.

This of course brings up the question of the purpose of the fiction disclaimer in general, and its legal requirements and ramifications.

Not all fiction novels contain such a disclaimer.  I don't know the statistics, but I've got a 1983 printing of Brave New World sitting here on the desk and a quick check reveals no such disclaimer.  A David and Leigh Eddings book, The Redemption of Althalus, that I randomly took down from a shelf also contains no such disclaimer.

Apparently the disclaimer originated because of the movie Rasputin and the Empress.  The Russian Princess Youssoupoff sued MGM for libel over the portrayal of a character that she claimed (and the court agreed) was based on her.  The movie was released in 1932, the court case occurred in 1934, but I can't find what movie was the first to use the disclaimer.  It may have been Rasputin and the Empress (they pulled the movie for several years) though I can find no reference to it.

So some form of fictional disclaimer tends to be tacked onto movies and books in order to prevent the author(s) from being sued.  Does this really work?  Well, the answer is somewhat.

At this point we are venturing into matters of law, which naturally varying from country to country.  What follows is mostly US law, since other western libel laws are similar enough; that or you are more likely to get sued by someone from the US.

The problem is that if people who know the plaintiff would identify a character with them, then argument can be made that, fictional disclaimer or no, the defendent has defamed them.  It doesn't have to be a match, but the differences do have to be fairly minimal.
Once you've got the match, you then have to demonstrate that it's not a satire or parody, and, generally speaking, you have to demonstrate that it's not actually true (I'm not even going to touch the sue-for-bad-restaurant-review fiasco).

Still, if you defame a vampire, they're not going to take you to court.
They'll just rip your throat out.