No, I know exactly what Legacy of the Void is. Vengeance of the Void was a bogus leak about the next expansion at the time returning to Outland and A'dal going evil. The "news" was broken by MMOwned.com and everyone ate it up because it was a faked document using Activision Blizzard's letterhead.
Here is a link with all the bogus docs.
The point here is that maybe the whole "The Dark Below" trademark app is legit or maybe it isn't. The doubt is reasonable enough to go either way since the "Corgis Unleashed" app pretty much showed how documents can be faked and cast doubt on the authenticity of the first announcement but the possibility remains that this could be a red herring thrown out for damage control. Nobody will know for sure until Blizzard decides to make an announcement on it.