It will work as it did prior to patch 2.1.0 meaning that 4 vials will drop per kill. On the release of Black Temple, it will be switched to multi-drop and it will then drop for everyone.
We are keen on having it this way so people can progress and build their path towards Mount Hyjal. The content is done, it was tested privately by multiple guilds which were recruited through public announcement on a first come - first served basis as seen
here and our QA team.
We are still considering this and this is not set in stone, it can go either way. We will announce it appropriately WHEN decided.