This fight has been bugged since release. If you pull Mayong Mistmoore the entire room of vampires will attack. If your raid force is geared for it you can tank every thing. How ever if your raid force is not geared, Then this is what I would recommend.

Have every one stand up the stairs on the throne and wait. Let all the mobs respawn. Once all mobs have respawned start killing quick. Once you kill a mob you will be on a respawn timer. So clear as much as you can for a clean pull on Mayong Mistmoore.

Have the tanks start pulling the Vampires that are not linked to Mayong Mistmoore. Burn and keep moving and pulling. Once you get to the vampires that are on the floor you will need to have players that are not grouped with you and that have encounter locks turned off start pulling the vampires to the raid force. Tanks be ready! Burn them quick before Mayong activates. Have healers be ready to rez if those players die.

Having a conjurer is good for Call of Hero on this fight. Clear as much as you can.

Once the area is clear or you wipe and feel comfortable killing Mayong, I would recommend fighting him at the entrance of the zone.