>> Implying Miku Hatsune isn't performing at Anime Expo.

So wait, this thing is like Microsoft Sam singing? They pump some lyrics into a program and it outputs them in autotune?

Pretty much, although it can be more complex than just that. Quick snippet from wikipedia:

The software enables users to synthesize singing by typing in lyrics and melody. It uses synthesizing technology with specially recorded vocals of voice actors or singers. To create a song, the user must input the melody and lyrics. A piano roll type interface is used to input the melody and the lyrics can be entered on each note. The software can change the stress of the pronunciations, add effects such as vibrato, or change the dynamics and tone of the voice. Each Vocaloid is sold as "a singer in a box" designed to act as a replacement for an actual singer.