Common Strategy:
He will become agro when Venekor is engaged. He will not attack, however, until he gets line of site of someone in the raid party.
Suggest fighting Venekor down to 10% or so, and then having someone trigger Varanak. This is the safest way to defeat both encounters (assuming you even want to kill Varanak for loot).
You can fight Venekor without Varanak by pulling the dragon behind one of the temples, out of sight of Varanak.
More information is needed.