Hatsune Miku was not intended to sing in English but by manipulating Japanese phonemes, many Vocaloid users simulated ‘English’ songs in her voice. However, Japanese phonemes lack sounds that are required in English pronunciation. The attempt to simulate English using Japanese phonemes forms a dialect known in common slang as ‘Engrish’. In Japanese Vocaloids, main signs of simulated ‘Engrish’ are the l/r sounds that are not distinguished in Japanese and the ‘th’ sound absent in Japanese phonetics (usually replaced by ‘ch’ sound).
Below is an example of Hatsune Miku singing in "Engrish".
As of Vocaloid 3, an English vocal now exists for the Vocaloid which will effectively produce English.