I've done the quest for the cure and saved/killed Count Dracula's wife, but now the witch won't offer me the second potion she promised. All I can ask her about is the cure for vampirism, and she says "Once I have all the ingredients I need, I'll fashion a cure for you. This is a delicate matter, and cannot be rushed."

Um...I've given her the ingredients. Do I have to get a second set? If so, I've already killed the super ubar powerful vampire, where do I find another?

So, you mean that you have finished the quest. The quest does not require that you have cured your Vampirism already, and while there is the offer of getting more potions if needed, however, they would not be, unless the game is modded. This is becuse, by default, if you do the quest, and take the cure, becoming a Vampire again is impossible(as the value the game checks for when a PC has the required disease, is a "0", which allows the progression to Vampirism. After the cure is taken, the value is now a "-1", and as such, the disease's does not progress to Vampirism.)

However, if you mean a second potion as in the one for the characer, you should be given both one for Hassildor's Wife, and for yourself.

Turns out I'm an idiot. Potion in inventory, thanks