Well, that's got to be one of the most annoying quests I've come across yet.
When you got to the muddy water it said to pick your path carefully as the vial leaks.
So I was filling it then trying to run/walk all kinds of different routes back to the quest giver.
After failing many time I checked it out on Wowhead. Apparently you need to keep filling the vial until it is full then it stoppers itself.
Why couldn't Blizztards just freakin' say that?
On the upside Midsummer has just started and I am micrometres away from level 88 ...
... and I have now just hit 88 :)