Wondered the same thing and received mixed views from guildies and friends. If I went there at 78 I would definately have needed a lot more healing (playing shaman).
Went on wowhead to the hyjal and vashj'jir pages and looked at quests, only 3 (iirc) quests in hyjal required 78 and they were 2nd in a chain from a lvl 80 quest.
Just a long insightful comment saying no, its not possible :>