Vial of Phlogiston

The Vial of Phlogiston drops from Roogug, a named Quilboar in Razorfen Kraul if you are on the Horde quest or on the Alliance quest. Razorfen Kraul can be found just north of the Great Lift in The Barrens on the west side.

Source
This item drops from Roogug in the instance Razorfen Kraul.