According to Grove, the name Vox Angelica was first used around 1750 for a small 2' reed stop. Williams adds that it was used for a variety of soft high-pitched reed stops in 18th century Germany. In the 19th century the name was used for various soft-toned flue stops. It is considered by some writers to be the softest stop in the organ. It is often made as a type of Dulciana, and may or may not be a celeste.
Osiris contains 75 examples of Vox Angelica, all at 8' pitch except for a handful at 4'. Only two of them are reeds. The same source contains five examples of Voix Angelique, all flues. No examples are known of Engelstimme or Flauto Angelico. Contributions welcome.