/jp/ sent me here. Not sure which will answer it better.
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I have recently began to study linguistics, and I know a little about Vocaloids. I know they use a human-created database of different phonemes, and use combinations of those phonemes to emulate human singing. Some Vocaloids like Miku and GUMI can sing in Japanese, and some like Luka can sing in English as well.

What my question is, is: what is preventing the voice "actors" for Vocaloids to simply make all the phonemes in the International Phonetic Alphabet, therefore making them theoretically capable of singing in any language? Is there some sort of software limitation besides just the phonetic database? I wouldn't know if there was; I've never used the software.

I looked around and all I could find was a page on the Vocaloid wiki about phonetics, with no real tie-in to how it's implemented into the Vocaloid software: http://vocaloid.wikia.com/wiki/Phonology_and_Phonetics_for_Vocaloid_users