How do you get to Vashj'ir?

Answer 1

For Alliance

At the East end of Stormwind there is an island in a lake with all manner of strange monuments and a Shaman in the middle. There's also a mage there opening a portal. One of these portals will lead to Vashj'ir.

Also, to be sure, you can click on the Hero's Call board in SW once you have instaleled Cataclysm, and it will refer you via a quest.

Note that it is not possible to go to Vashj'ir or any of the level 81+ zones before you have Cataclysm installed.

For Horde

Go to the hero call board and accept the quest for Vashj'ir, then follow the quest tracker until you reach the docks outside of Org. Go to the end of the docks and wait. Some elites will show up in gold gear and talk nonsense. After that a boat will arrive and take you to the island.

If you have already done some quest there you should have a portal active in org.