Where in Constitution is VPOTUS not eligible because of previous service as POTUS?
12th Amendment.
Obama could become Acting President if he became Speaker of the House and the President and Vice President were both removed, killed, or decided to resign without time for a new Vice President to be chosen. That's not going to happen.
Constitution says VPOTUS must be eligible to be POTUS. Obozo is not eligible to be POTUS (again) therefore is not eligible to be VPOTUS. QED