Rinse and pick over grapes, remove stems and dry completely.
To toast pecans: Preheat oven to 350°F. Spread pecans in a single layer on a baking pan. Toast for 6-8 minutes until lightly golden and fragrant. Let cool, then chop.
To make the dressing: Add Neufchatel, sour cream, sugar and vanilla to a large mixing bowl. Cream together using an electric mixer for 2 minutes or until smooth and fully combined.
Use a large spatula or spoon to gently fold the grapes and toasted pecans into the dressing until evenly coated. (Reserve 3 tablespoons pecans for garnishing)
Place into an airtight container and chill for at least 4 hours or overnight.
Sprinkle the top with brown sugar and garnish with reserved pecans just before serving.
Store leftovers in an airtight container chilled for up to 3 days.