The report has been confirmed by the AJC's Tony Barnhart, and may be considered fact at this point since Barnhart doesn't report anything he doesn't hear from the horse's mouth itself. (Or failing that, he at least talks to the jockey.)
The most insane offseason in SEC history continues with the resignation of Vanderbilt's most successful coach in recent history. Johnson did not exactly turn Vandy into a power with a 29-66 record at the school, but he did turn them into a competitive side in the SEC and led them to a Music City Bowl victory in 2008 while improving their recruiting, expanding facilities, and engineering several major upsets along the way like Vanderbilt's victory over Georgia at home in 2006.
The suddenness of the departure and the odd timing points to the usual suspects: health reasons, an unanticipated personal reason, and the least likely of all, personal scandal (since Johnson is a taciturn, straight-arrow type with an unassailable reputation).