Who are doing it?

There is no proof on whether there is a single force behind the takedown campaign and if so, what force it is. One does have to note that the campaign affects primarity videos for non-Japanese audiences (such as translations) and comes shortly after a surge in Vocaloid popularity outside Japan.

Some users on Japanese forums have suggested Korean involvement. To the best of the writer's knowledge, no proof has been demonstrated, at least in English. It is known that some users on Japanese forums are biased against Koreans for historical and political reasons that are very hard for Western fans to understand.

Personally, the writer would advise against publicly implicating any company, person, or country without posting proof, as in the worst case a lawsuit might result. Even if proof is available, be aware that impostors exist.

"Media Interactive": impostor?
One of the names of alleged copyright owners was "Media Interactive". A Japanese company of that name does exist. It has issues the following information on its website at http://mediainteractive.co.jp/ {C}translation by "sugar_gem" at http://vocaloidotaku.net/index.php?/topic/28016-media-interactive-inc-fake-copyright-holder-deletes-vocaloid-videos/page__view__findpost__p__718786:

'''Some people inquired whether our company was concerned in deleting Hatsune Miku videos on Youtube. There is no fact that we requested deleting or something. We know nothing about that problem.'''

According to a Google search, two other company named Media Interactive exist:

http://www.mediainteractive.cl/ in Peru

http://www.mediainteractive.co.nz/ in New Zealand

None of these seem to have to do with either music or Japan. So, the "Media Interactive" claims are likely to be from an impostor.

And this means that all other claims might be from impostors, too!

Obvious pseudonyms?
Variations of "Junichi Sasa" and "Tempura" in Japanese are often used.

It is worth noting that "純一" may be read as either Jun'ichi, Yoshikazu, or in less common circumstances, Sumikazu.

They are most likely to be pseudonyms.

"The Music Company" was reported to be involved. This is also likely to be a pseudonym; at least we were not able to find a company with that name that could be linked to Vocaloid work.

Sony (various divisions).
While Sony is known to be on the side of strict copyright and was inmplicated in a copyright-related scandal with rootkits on audio CDs (see http://en.wikipedia.org/wiki/Sony_BMG_copy_protection_rootkit_scandal ), it is likely that any claims originating from Sony are genuine, as it is a major corporation with a strong legal team. Sony owns PlayStation and therefore can probably claim anything involving Project DIVA. (Please be aware this is all personal opinion and NOT legal advise). We have no idea whether anyone could impersonate Sony.

TOYSFACTORY
A real Japanese music publisher which did release Vocaloid songs. Notably, in this case http://www.google.com/support/forum/p/youtube/thread?tid=736511a6dc07b3a5&hl=en it was found that the copyright to the song was indeed owned by TOYSFACTORY.

There was a tweet (from the famous producer kz http://vocaloid.wikia.com/wiki/Kz ) that TOYSFACTORY made some claims by mistake and is rescinding them: https://twitter.com/#!/kz_lt/status/161383256602845184

ELECTROCUTICA
a real Japanese music publisher which did release Vocaloid songs.They have clear rules for re-uploads: http://electrocutica.com/faq#oversea.

It may be that the rules were violated and the takedowns were thus genuine. But it may also be that impostors have used ELECTROCUTICA's name.

It would be really good to contact ELECTROCUTICA to make sure, but we need Japanese language help for this.

"Shenzhen Eri": impostor?
One of the names of alleged copyright owners is "Shenzhen Eri". The real Shenzhen Eri is a Chinese company that manufactures electronic components. It is highly unlikely that the company would hold any Vocaloid related copyrights. Here is a Website advertising this company's products: http://eri-tech.en.alibaba.com/