On a second thought:
How to vote (cont'd):
> click on the big red button (投票する)
> click on the first link (投票用ID[…])
> type in your email address and click on the blue button below (送信)
> you will receive your ID
> go to the previous page
> now click on the second link (キャラ部門)
> type in your ID in the first field
> (optional) type in your name in the second field
> click on 未選択 under 投票.1 (counts twice and is mandatory)
> select Youmu (魂魄妖夢)
> (optional) select other characters in 投票.2~5
> click on the blue button
If there isn't any red text on the next page, your selection works. Now, click on the grey button below to submit your vote.
If you get redirected to another page with a twitter share button, your vote has been accepted.