First off, let me say that I really like the wall as it is. The minor changes I am going to suggest are exactly that --- minor.
As others have mentioned, her shadow is slightly off. It seems slightly too thin for Vivi's body. Additionally, it seems like the shadow has been projected onto a flat, smooth surface as opposed to a sand-strewn beach path. If the shadow had some variations in height as if it were being cast on uneven ground, then it would help blend the bg and Vivi together.
The grass has a sense of blurring caused by motion and so should Vivi's skirt hem. The moving parts of the vector, like her skirt, hair, and the yellow ribbon, should show the same sense of motion thru blurring that you applied to the grass blades.
The last thing I would say is to round off the hill tops in the background. The jagged peaks look a little unnatural.
All in all I think this is a lovely wallpaper and I want to thank you for sharing it with everyone to see.
