You don't need Vanille's EX, only her 35 CP /w 15 passive. With a Quistis friend she's usually hitting that HP threshold on both of her abilities and increasing everyone's damage by removing Ifrit's DEF buff and applying her Deprotect.
Her EX will make everything go faster obviously but it's far from required. Celes is the safe way if you don't have strong characters with her but it'll take a lot longer for each clear.