Melt the butter in a medium nonstick saucepan over medium heat. Add the condensed milk and vanilla and stir to combine. Reduce the heat to medium-low and cook, stirring occasionally, until the mixture turns to a light caramel color (240 degrees F on a candy thermometer), about 25 minutes. Turn off the heat and incorporate the pecans. Let cool for about 8 minutes. Roll into 2-ounce pieces and wrap with parchment paper.