Every team is going to have tight games at some point. Plenty of teams are going to have very tight games against teams they absolutely shouldn't struggle against.
Make no mistake about it, Vanderbilt has one of the better defenses in the conference. They have one of the best running backs in the conference. Auburn still should have handled them without too much of an issue, but when the Tigers play a full half without the starting quarterback while also missing the leading receiver and two linebackers for basically the entire game, then the dynamic of any game will change.