In a large bowl or bowl of your stand mixer, whisk together warm milk, instant/rapidrise dry yeast, sugar, 8 tablespoons of the melted butter, vanilla, eggs and salt. Slowly mix in flour, 1 cup at a time, with a wooden spoon or spatula (or paddle attachment on your stand mixer on low speed) until the flour is incorporated and the dough starts to come together (dough will be tacky to the touch).
Then, knead dough, by hand on a lightly floured surface a few times (or on medium speed in your mixer with the dough hook) until it is smooth and and elastic, about 1 minute. Form the dough into a ball and place in a bowl coated with vegetable oil. Roll dough in bowl to coat in oil. Cover with a towel and place it in a warm, draft-free place to rise for about 1-2 hours, or until doubled in size.
Meanwhile, spray the bottom of a 9x13 inch baking dish with nonstick spray and prepare the filling. Mix together the brown sugar, vanilla, white sugar, cinnamon. Let butter sit out to come to room temperature and soften.Once the dough has doubled in size, turn it out onto a lightly floured work surface and, using a rolling pin, roll into a 12x18 inch rectangle. Spread the softened butter all over the top of the dough and sprinkle cinnamon sugar mixture on top, so entire surface is fully covered.Roll the dough up tightly, lengthwise, so you have one long 18" log. Cut dough into 12 equal pieces. Arrange them in the prepared baking pan, cut sides up.Loosely cover and let rise a second time in a warm, draft-free place, until they are puffy and doubled in size, about 1 hour. (To speed up the dough rising process, heat your oven to 200 F degrees and then, turn off the oven heat and let your rolls rise quickly in the warm oven until puffy and doubled in size.)
After the rolls have risen, preheat the oven to 350°F. Bake for about 20-25 minutes, until they are golden brown.