Place doll where she can lay undisturbed overnight, then coat the inside of the vagina around the area of the split with a thin layer of clear silicone.

Make sure that the doll is positioned so that the split closes by itself, in as close to the factory pour position as possible. The application of a thin layer of silicone on the inside of the vagina will reinforce the areas next to the split, and help to prevent more splits from occuring.