Her spirit is still with you... I'm sorry you feel the need for further burial. Perhaps part of the incompleteness stems from the (seemingly) lack of control of her burial? She is not there in the vault..she is no longer trapped in this world at all. (((WW)))

Can you go ahead and make that tree her place? Do you have any samples of things she loved, or perhaps things that were hers that you could part with? Music she loved, games she played... anything? Then take them to the tree, and give them... take cornmeal for the tree, as a way to tell it thank you for taking on that task...