Remove stems from grapes and rinse the grapes in cold water. Set the grapes in a drainer in the sink.
In a large mixing bowl, beat the cream cheese, sour cream, granulated sugar and vanilla until smooth. Blot the grapes dry with paper towels (you don't want to make the dressing watery.) Stir the grapes into the dressing until combined. Cover and refrigerate until ready to serve.
For the topping, chop a heaping ⅔ cup of shelled walnuts. Measure out 5 ounces of the chopped nuts and add them to a small bowl. Add the brown sugar and salt, and toss with a fork.
To serve, spoon the grape salad into stemmed glasses, ramekins, or small dessert bowls. Us a fork to sprinkle on some topping for each serving. (Or, leave the grape salad in a large bowl with some topping sprinkled on, and have extra topping in a small bowl on the side for people to sprinkle on.) Eat with dessert forks, since the grapes can be large.
Store leftovers covered in the refrigerator for up to four days.
Since grapes are a choking hazard, plan on cutting them in half for any children 5 and under.
The topping amount is perfect if you are scooping out even portions in stemmed glasses or individual bowls. Consider making extra topping if you will be leaving it on the buffet table for people to serve themselves. Use a ratio of 2 parts chopped walnuts to one part brown sugar. Add salt to taste.
Plain 2% Greek yogurt is not bad in place of the sour cream, but it does have an extra tang. I'd use it for making this to snack on at home.