Wash the red and green grapes under cold water, remove and discard any stems, then pat dry with paper towels; let them sit on the towels to finish draining.
If the walnut pieces are larger than bite-size, chop them to your preferred size.
Toast the walnuts: heat a dry skillet over medium to medium-high heat, add the walnuts in a single layer, and cook 3–6 minutes, stirring once or twice, until they are fragrant and lightly browned. Immediately transfer the walnuts to a cool plate to stop the cooking.
In a medium bowl, beat the room-temperature cream cheese until smooth.
Add the sour cream, powdered sugar, and lemon juice to the cream cheese and beat until the mixture is well blended and smooth.
Stir in the vanilla extract until fully incorporated.
Add all the grapes and the toasted walnuts to the bowl with the dressing.
Gently fold the grapes and walnuts into the dressing until everything is evenly coated.
Transfer the grape salad to a covered bowl or container and refrigerate for about 1 hour before serving.