Coat a rimmed baking sheet with cooking spray. Heat oil in a large pot over medium-high heat until hot but not smoking. Add popcorn, and cover. Cook, shaking pot frequently, until corn has finished popping. Transfer to a large bowl.
Heat butter, brown sugar, corn syrup, and salt in a saucepan over medium-high heat, stirring until sugar has dissolved and butter has melted. Clip a candy thermometer to side of pan, and cook until mixture reaches 290. Remove from heat, and stir in vanilla and baking soda.
Drizzle syrup evenly over popcorn, and toss to coat. Spread popcorn on prepared baking sheet, and sprinkle with fleur de sel. Let cool completely, then separate popcorn into pieces. Popcorn can be stored in airtight containers for up to 3 days.
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