Spread the sugar in an even layer over the bottom of a large, heavy-bottomed saucepan. Place over medium-low heat, watching carefully. When the sugar begins to liquefy around the edges, use a heatproof spatula to gently stir it towards the center. Continue stirring very gently until all the sugar is melted, taking care not to over stir.
Measure out the heavy cream in a liquid measuring cup and scrape the seeds from the vanilla bean into the cream. Set aside. Once the caramel reaches a deep amber color, immediately remove the sauce pan from the heat. (To test the color, spoon a drop onto a white plate or bowl.) Carefully whisk in half of the heavy cream along with the vanilla bean seeds. The mixture will steam and bubble violently. Stir until the cream is well incorporated, then whisk in the remaining cream.
Stir in the salt and the vanilla. If any sugar has hardened, place the saucepan over low heat and whisk until smooth.
Store in an airtight container in the refrigerator. If needed, it can be rewarmed in the microwave or in a saucepan over low heat.