Most fans expressed that they would like a vocaloid anime that focused on the Crypton Future Media vocaloids. However, Crypton has stated that they will never release an anime because they do not want to impede on creators ideas of the vocaloids. In laymen term Crypton doesn't want an anime because it would ruin the Promotional videos and Miku Miku Dance works that have been created.
This does not rule out other studios getting animes for their Vocaloids.
At this point in time, focus on Vocaloid is on Vocaloid 3, so a promotional animation would likely focus on those particular Vocaloids. Also, another thing to consider is that most of the studios work individually from each other. Therefore, the issue is that a anime would focus on a very select few Vocaloids and depending on what studio ran it would mean which Vocaloids ended up in it. For example, if CFM ran it, this would be the only time you see fan mades and othe guest appearances from non-offical Vocaloid sources.
If Crypton were to make an anime, I would assume that all of their official Vocaloids (Miku, Meiko, Kaito, Luka, Rin, Len) would be in it, along with Neru, Haku, and maybe Teto. But they said that they're not making an anime because it would limit the creativity of their user base. For example, if they made Rin an innocent little girl, that would ruin the song "Daughter of Evil", because Rin is a tyrannical princess in the song. Or if they paired Meiko and Kaito as a couple, that would ruin the song "Cendrillon", since it focuses on Miku and Kaito.