How to obtain Vial of Distilled Mana
In order to obtain a Vial of Distilled Mana you'll need some Sapphires, Poison Vials and an Enchanter with the spell Distill Mana or Mass Distill Mana. Sapphires are sold by a variety of different Jeweler Merchants, there's usually one of these in every home town as well as PoK.
Poison Vials are the more difficult of the two items to obtain, they're either created through a Pottery combine or purchased from a vendor. If Plane of Knowledge is live on your server you'll find the Poison Vials sold from vendors in this zone. Prior to PoP you'll likely have to do some traveling to find a vendor that sells Poison Vials. List of Poison Vial Vendors
Spell: Distill Mana
2 Sapphire + 1 Poison Vial = 1 Vial of Distilled Mana
Spell: Mass Distill Mana
10 Sapphire + 5 Poison Vial = 5 Vial of Distilled Mana
As aforementioned you can also make Poison Vials through a Pottery combine. This method is extremely inefficient and time consuming, however, each of the different combines to create a Potion Vial count towards 350 Pottery. Due to this I have compiled a complete list of all the combines which you can find here: Pottery Poison Vial Combines.