Heat oven to 250 degrees F. Spray a large sheet pan with nonstick cooking spray.
In a large mixing bowl, combine cereal, pretzels and pecans.
In a separate microwave-safe bowl, combine brown sugar, corn syrup and butter. Microwave until butter melts, about 1 minute. Continue microwaving in 30 second increments, stirring each time, until mixture is smooth and begins to boil. Stir in vanilla, baking soda and salt.
Pour syrup over cereal mixture. Stir to coat. (I find it easier to mix this by hand, so I let it cool slightly and then mix.)
Spread cereal over sheet pan and bake 1 hour, stirring every 15 minutes.
Cool completely. Once cooled, gently break cereal apart. Store, covered, at room temperature for up to a week.