"Vampyre" is an older spelling of the word 'vampire' and was used predominantly during the 1700 and 1800s. Its use in the group's title is to provide a more "Gothic" feel. And because I like it that way.
Let's go back and find some origins of the word vampyr or vampyre.
1734 -- The word "vampyre" enters the English language in translations of German accounts of European waves of vampire hysteria.
1810 -- Reports of sheep being killed by having their jugular veins cut and their blood drained circulated through northern England. --"The Vampyre," a poem by John Stagg, is published.
1819 -- John Polidori's "The Vampyre" is published in the April issue of New Monthly.
1829 -- Heinrich Marschner's opera, Der Vampyr, based on Nodier's story, opens in Liepzig.
1932 -- The movie Vampyr, directed by Carl Theodor Dreyer, is released.
QUOTE
The word vampire (or vampyre) arrived in the English language with two 1732 publications: the March translation of a report by the investigators looking into the case of Arnold Paole of Meduegna and the May release of the article "Poltitical Vampires."
The latin genus Vampyrus, note the Y.
Elizabeth Miller has some interesting reading and is a well known Dracula scholar.
http://www.ucs.mun.ca/~emiller/rwbdinn.htm
The oxford dictionary being used is likely a modern adaptation and surely different from the 1700 version dictionaries, much like Olde English differs from our language today. Words change their spelling as they become widely accepted.
There are many who use the word "vampire" for every stereotypical example of "vampire kiddies." It isn't depending on whether they spell it with a 'y' or with an 'i'.
Which is more accurate or more important ? That should be entirely yours to decide. Which word captures the essence or feel of the definition of what you are trying to convey? -- for this truly is what matters most, in my opinion. People today still grow their hair long and wear tie-dye and peace symbol necklaces. People today still dress punk or gothic or 80s or other cultural or era influenced style. Use the words however they fit best in your own definition and avoid listening to someone else trying to stereotype it for you. They are the real posers. The ones grabbing a word and defining it for the entire culture as-if it can only be that way, and no other.