What if instead of killing that Vasto Lord she sealed it withing herself "Kushina style" so it was inside her while she was pregnant. The idea of killing a VL that even a captain couldn't beat seems a bit much but maybe using some ancient Quincy technique she could seal it.

Also I was wondering why Isshin aged but I guess it has something to do with the artificial body he was given


- On a side note Kukaku Shiba must be the younger sister of Isshin and I bet she lost her arm fighting that Vasto Lord thing.