While he may not have been allowed to hold onto the documents after he left the position of VP (should have returned them when he left, yeah, I said it), he WAS allowed to see them and determine the classification. Found the EO for you, here's the link.............
obamawhitehouse.archives.gov
Here is the part where he has classification authority:
Sec. 1.3. Classification Authority. (a) The authority to classify information originally may be exercised only by:
(1) the President and the Vice President;
(2) agency heads and officials designated by the President; and
(3) United States Government officials delegated this authority pursuant to paragraph (c) of this section.
And here is the part that includes his being able to declassify things (if he was the one that made it classified)...........
PART 3 -- DECLASSIFICATION AND DOWNGRADING
Sec. 3.1. Authority for Declassification. (a) Information shall be declassified as soon as it no longer meets the standards for classification under this order.
(b) Information shall be declassified or downgraded by:
(1) the official who authorized the original classification, if that official is still serving in the same position and has original classification authority;
(2) the originator's current successor in function, if that individual has original classification authority;
These are just portions of the EO, which is why I posted a link to the full thing. Would be too long to copy it into a post.