The stadium is about 1/3 full now from the look of it. If you had seen the rain and lightning at the time that kickoff was scheduled for, you would understand why most of the crowd went home.
We had a similar weather delay vs Montana in 2011 and not many left....the better explanation for the small crowd is that Vanderbilt simply doesn't have very many fans and the fans they do have are not emotionally invested in the football program. They always struggle to fill their 40,000 seat stadium when they play an opponent that doesn't bring very many fans with them.