Because Gumi is a Japanese VOCALOID, she will only recognize valid Japanese Romaji, Hiragana, or Katakana. 'Lo' is not a Japanese syllable, and therefore is not recognized by Gumi (And will not be by any JPloids, for that matter). The reason she sings 'a' instead is simply because thats what JPloids default to when an invalid lyric is entered. The user word dictionary also only works with Engloids. (and its annoying as heck) Also keep in mind that the phonemes listed by the user word dictionary are only for Engloids as well.
This is the FAQ page for the official VOCALOID website run by Yamaha Corporation. About 'Please advise how to import English user dictionary file of VOCALOID2 into. Vocaloid Download English VIRTUAL VOCAL SOFTWARE. VOCALOID TM 3 Megpoid English is the virtual vocal software based. Crypton Future Media, Inc. Related stories See all.
Cmi8788 Driver Windows 10 more. JPloids use a different set of phonemes. There are couple alternate ways to do it, but try using [do] instead of [lo] with some tweaking on velocity. Or put short(either 16th or 32nd) [do] before [o]. It should give you 'lo'-ish sound. I got this pronounciation fact from this article.(it's in jpn btw) Generally speaking, focus on 'how it's supposed to sound like' than 'how it's written'. And if there's no consonant you want from your library, think of closest consonants.
And mix with other consonants/vowels. Or use some rests in between to adjust the nuance. By using combination consonant method, you should be able to simulate most of English consonants.