During Vezax hard mode you cannot regain mana apart from the Fel Mana Potions which has been listed before. There is one other way to gain mana.
Using Innervate and then Divine Intervention on someone will cause them to drop combat, remove the debuff and give them back mana. It is very useful when you have two healers oom, for example a Druid and a Paladin and the paladin is soulstoned, the druid Innervates himself, gets DI'd by the Paladin, returns to full mana, removes DI and is good to go, and the paladin resurrects with soulstone to regain mana from the ressurection.
Good Luck!