Vocal loops are a common occurance in music, especially in computer-produced electronica style songs where it is easy to manipulate human voice digitally. It is a reasonably simple process though, and with a few sound editing tools it is very possible to create them manually as well. Here's how it's done:

1. Singer sings a steady note.
2. A portion of the sound is recorded on DAT, tape, or other medium.
3. The start and end of the recorded tone are spliced to each other. In a digital medium, this can all be done on computer without any manual manipulation.

That's it. The tone can now be played for as long as is necessary, and the "singer" will never waver or tire. The tape can be sped up or slowed down to produce different notes, which has the same effect as pitch shifting and eliminates the need for a time stretch to lengthen or shorten the sound.