Rinse both the red and green grapes thoroughly under cool water to remove any dirt or residue. Spread them out on a clean paper towel or kitchen towel to air dry completely. Excess water left on the grapes will water down your creamy dressing later.
Spread the chopped pecans on a small baking sheet and place them in a 350°F oven for about 5 to 7 minutes. Watch closely and pull them out once they smell fragrant and toasty. Let them cool fully before using them in the topping.
In a large bowl, whisk together the softened cream cheese, sour cream, granulated sugar, and vanilla extract. Keep mixing until the mixture turns silky smooth with no lumps left. This dressing is really the heart of the whole dish.
Add both the red and green grapes into the bowl with your creamy dressing. Gently fold everything together using a spatula, making sure every grape gets coated. Be gentle here so you don't bruise or split the grapes.
Transfer the coated grapes into your serving bowl and cover it with plastic wrap. Pop it into the refrigerator for at least 30 minutes, or until you're ready to serve. Chilling helps the flavors meld and keeps the salad refreshingly cool.
Right before serving, sprinkle the packed brown sugar evenly over the top of the salad. Follow that with a generous scattering of your toasted, cooled pecans. Serve immediately for the best texture and crunch.