Well, I would write only the name of the singer of that Vocaloid cover song in artist.
Because in that case, He/She is the real singer and not the creator of the song anyway.
I don't understand, Miku is the real singer of the original song too, why don't write her name as artist and write the singer's name for a cover, it's illogical.
What bothers me, is that with my 5000 beatmaps, it's really not handy to find a song. When I search Vocaloid songs, I'm going to the name of the Vocaloid in the list, and I find all her songs. I never play Vocaloid songs with other artist names 'cause they are lost in the thousands of beatmaps....
Moreover, know the creator of the song is secondary for me, I think the most important information is the Vocaloid who sings.
Since I'm on Osu, the "mapper"/creator of the song was written in the "Source" field of the map, I think it's really much better like that and really much handy to find the map. Example :
http://osu.ppy.sh/s/19887It' just my opinion, but I hope you will change your point of view on this question, 'cause that's really boring to find a map in those conditions...