We’re working on getting Vanishing Dyes for Legendary items into 1.0.7. We intend for them to have the same slot restrictions as dyes traditionally have, so you’ll be able to use Vanishing Dye on say, your Legendary shoulders or helmet, but not items such as your weapons.
We hope this change will be available for testing on the PTR in a future build, but that may not possible. We’ll keep you updated. :)
Blizzard is also working on making every dye work for Legendary Items
We’re definitely working on making all dyes work for Legendary items. It’s not a quick-fix, though; as Fearophobias surmised, our artists actually need to go back and redo the textures on all the Legendaries in order to make them dye-compatible.
Since we know this process will take some time, we wanted to make sure that players could at least use Vanishing Dye for now (it was the most-requested dye for Legendaries that we’ve seen to date).