Combine sugar, syrup, butter, and whipping cream in a large Dutch oven. Cook over medium heat, stirring constantly, until sugar dissolves. Cover and continue to cook over medium heat 2 to 3 minutes to wash down sugar crystals from sides of pan. Uncover and cook, stirring frequently, until mixture reaches firm ball stage (244°).
Remove from heat. Stir in vanilla and pecans, mixing well. Working rapidly, spread mixture in a buttered 15- x 10- x 1-inch jellyroll pan. Cool completely before cutting into 1- inch squares. Wrap each piece in plastic wrap. Store in airtight containers.