Stir pecans, sugar, syrup, and salt together in a 1 1/2 quart glass bowl. Microwave on HIGH 7 to 8 minutes, stirring well after 4 minutes. At the end of 8 minutes, add butter and vanilla. Blend well. Return to microwave and cook on HIGH 1 to 2 minutes more. Remove and add baking soda and gently stir until mixture is light and foamy. Pour onto lightly greased cookie sheet and let cool 30 minutes to 1 hour. Break into pieces and store in airtight container.