Unknown Vocaloid artists
Sometimes it’s hard to find information about a particular artist, especially if all you have is the name and no leads such as mylist/twitter, or the linked profile is deleted. If the artist in question is a Vocaloid user and it doesn’t look like they’re on VocaDB yet, you should add an entry for that artist, but leave it as draft. Generally this is better than adding the artist as just a name (extra artist) for the song/album as it allows linking songs/albums by the same artist together.
Always try to find as much information about the artist, if at all possible. Often just Googling with the artist name, or mylist/twitter/other profile name is enough to find a blog or twitter channel. Remember, quality over quantity: it’s better to add a small number of well-researched entries rather than a huge number of songs with missing artists. Researching artists can take a lot of time, but it gets easier as your learn the techniques. Of course, knowing Japanese or using the appropriate tools helps a lot.
If there’s multiple unknown artists for a single song or album, you can credit them as extra artists, but in that case be sure to leave the entry status as draft and add the Entry_missing_artists tag.
Also remember that whenever you’re crediting an artist by name you need to specify the roles.
For Vocaloid covers of non-Vocaloid songs, whether you should add an entry for the artist depends on whether or not that’s the only Vocaloid cover of that artist’s songs. For example, many of ZUN‘s Touhou original songs have been covered with Vocaloids, that’s why we have an entry for ZUN, even though he’s not a Vocaloid artist himself. If only one or two of the artist’s songs have been covered with a Vocaloid and the artist itself isn’t related to Vocaloid in any way, generally it’s enough to credit that original artist as just a name (usually with the composer role), preferably with an explanation in the song notes. I’ve seen a lot of editors omit adding the artists in the artists list, but it’s preferable that the artist is mentioned there as well. If there’s no additional information you may even skip the notes. If the original (non-Vocaloid) song is on Youtube or NicoNico, you may link that in the external links section (don’t use the notes field or media list for that).
Most of the time it’s enough that the composer of the original song is credited, but crediting the lyricists and performers is recommended as well if the information is easily available, and especially if those artists are on VocaDB. Vocal performance of the original song does not need to be credited in the artists list, because that vocalist was obviously not involved with the making of the cover. However, often it’s the case that the original vocalist is more well known than the artist who composed the song, so mentioning the original vocalist is necessary. This should be done in the notes/description field. If there’s multiple Vocaloid covers that were originally sung by the same vocalist it’s ok to create a tag for that vocalist. This is one of the few cases where tags should be used for indicating an artist.
Note that, apart from the example above, unless there’s a reason otherwise, tags should not be used to credit original artists of non-Vocaloid originals. The artists list is for crediting artists, tags are for copyrights such as movies, games and TV series.