The artist is supposed to be an artist or artists implemented to a song. If original artist does not sing in current song, because it is sang by a Vocaloid, then his name belongs to tags, same about the vocaloid's name. To the artist, there belongs an artist/group which made that vocaloid version. If the song is not completed rework, but some kind of mix, then it shall be just
Original Artist - Title (someone's Mix), if the song is whole remade and there is nothing used from original artist, then original artist belongs to tags, artist shall be the one who remade it completely and the title depends on how he called it by himself. This applies to vocaloid, if original artist is implemented, but someone added vocaloid sing to it along with the original artist, then it would be
Original Artist & Person/Group who changed that song - Song title, or
Original Artist - Song title (Person/Group who changed the song Mix) or similar, if there is no implemented original artist, so it is a completely remade song, the artist will be in tags and it would be named
Person/Group who remade it - Song title and if you'd want to add vocaloid's name, then it belongs to tags along with original artist.
If it shall be added as a rule, then this might be used:
Artist is the person or people who acted in the song. If the song is remake of original, but original artist is still implemented, then artist shall be Original Artist & Remake Artist, or it shall be after the song title like Someone's Mix/Remake etc. If the song is complete remake, not including the original artist, then artist shall be Remake Artist and original artist belongs to tags. In case of Vocaloid songs, they might be added into tags, but not into artist unless the remake artists claims his songs to have it as an artist.