Good question. I think there's no canon explanation, but here goes my theory:
After the Ventus-Vanitas X-Blade ended in failure Xehanort realised he had to initiate a war between 13 darknesses and 7 lights. The 13 and 7 in KH3 weren't pure hearts, it was a matter of which side they were fighting on. Xehanort originally founded Org. XIII to find 13 suitable vessels for his heart to act as those 13 darknesses, even if they were not pure, so he must have known for a long time that pure darknesses and lights were not needed.
Vanitas always went along with Xehanort's plan without questioning it. If he thought Aqua could have replaced Ventus as the light half of the X-Blade, then Xehanort must have told him that replacing Ventus was a valid option if needed. Remember Ventus was comatose and about to die before Sora's heart joined his and saved him... Xehanort carried Ventus to Destiny Islands expecting him to die there. Back then he must have thought about replacing Ventus to keep his plans in motion.
If replacements were a plausible thing and he still wanted to use Vanitas as the dark half, then he knew from the beginning that pure hearts of light and darkness were not needed. An impure light like Aqua vs a pure darkness like Vanitas was probably a valid option, or perhaps Xehanort could extract Aqua's darkness and make her pure light if purity was truly required. In the end since Ventus recovered under Eraqus's care Xehanort went on with the original plan.
Maybe he wanted Ventus and Vanitas to be pure light and darkness in order to maintain a better balance between both forces, hoping it would simplify the process or make it less unstable. Xehanort admitted in Dream Drop Distance that he was a bit rushed back then and tried the "easy way". He was aging with not many years left to live unless he found a new vessel so it would make sense for him to try to simplify the process as much as possible.
