Bring first 3 ingredients to a boil, up to 236 degrees with candy thermometer. Stir so it doesn't burn. Put in butter, don't stir and leave on heat until melted. Once butter is melted take off heat and let it cool down to 150 degrees. Stir in vanilla and pecans until it is thick and it has hardened a little bit.
Put on wax paper and leave until hardened.