I recently just met someone who pronounced "Gumi" as "Gummy" needless to say, I changed the subject rather quickly...
I do feel, though, that if your favourite Vocaloid is Miku that there is nothing wrong with that, but if that's just because she's the only one you know about that it's not really being a real fan of Vocaloid as a whole, and mainly just a fan of Hatsune.
I don't have a problem with people like that, though, I myself only really liked Miku for a long time before I discovered Gumi and she became my favourite.