The Avatar can be considered as the highest possibility in a human body. In fact the Avatar is a descent of the Supreme Divine assuming a human form. Once that happens the form and the soul remains permanently upon earth to help man cross through the critical junctures which made the coming of the Avatar necessary. An Avatar does not die even after withdrawing from the physical body. He remains in contact with the earth in the subtle realms. A vibhuti on the other hand refers to a special manifestation of some aspect of the Divine in and through human nature. An Avatar has evidently gone through that experience of being a Vibhuti. To return as a Vibhuti would mean coming down a few steps and thereby becoming less effective. Yes there would be Vibhutis of the Avatar but that is different from the Avatar himself coming back as a vibhuti.
You will find a few talks on this subject in detail on auromaa.org
Affectionately,
Alok Da
