In seven of the 13 races, the incumbent won. In six out of those seven, the VEWB was flat or positive. In one race, the VEWB was slightly negative, and the incumbent also won (Dwight Eisenhower in '56).
According to the data, then, a flat-to-positive index is virtually required for an incumbent to win. Also, no one has ever won with an index as negative as Obama's is now. But a positive index doesn't guarantee victory. In the six of 13 races the incumbent lost, the VEWB was negative in three but positive in the others.
http://online.barrons.com/article/SB50001424052748703786004577221441309275760.html?mod=BOL_hpp_mag