Technically, Miku is already credited in the tag. If that person meant she should be tagged as the sole artist, then no, this is how it is on livetune's website, except without a space between 'feat.' and '初音ミク,' but that one redirects here anyway.
"(and writes, and has a say in the art direction)" How can a non living software program have a say in 'their' work? They can't, and besides it is the real artist (Livetune in this case) who has control over the virtual singer. So I agree with Thumkus, Livetune should get credit in full.
v But in the real world, music isn't credited to the producer, but the singer. Take the girl in my icon, for example. She doesn't produce her own music, she just sings (and writes, and has a say in the art direction). It's unfair but that's how it works. I personally keep note of all producers as an ID3 tag.
UPDATE! Our tags for Re:Package were NOT correct! I was cleaning up my tags when I found the "official" Hatsune Miku page; and yes, Re:Package is there as the official album!
Please correct your tags to [artist]初音ミク[/artist]
If you don't believe me, click the link & see for yourself!