Vashj’ir is one of the new leveling zones in World of Warcraft: Cataclysm. Eventually you’ll end up being able to travel there via portal in your capital city, but you’ll have to get there first and do some quests.
In order to receive the quest, you first need to be level 80 or higher and have Cataclysm installed. Once you’re level 80, you should receive a quest automatically called A Personal Summons when you enter your capital city.
After you go through the first part of this chain, check out the Hero’s Call Board in your capital city if you’re Alliance, or the Warchief’s Command Board if you’re Horde.
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You should find a quest to venture to Vashj’ir. It won’t take you directly there, but follow the quest chain and you will get there.