You can substitute an 8 ounce tub of whipped topping, but it only takes a few minutes to whip the cream and I think it tastes much better.
In a medium bowl, whip the cream with the vanilla and sugar until light and fluffy. Gently fold in half of the sour cream. Once incorporated, gently fold in the remaining sour cream. Add the Mandarin oranges, pineapple, maraschino cherries, and grapes. Gently stir to mix in.
Reserve 1/4 cup of the flaked coconut for serving. Gently stir in the remaining coconut, pecans and marshmallows. Chill for 1 hour prior to serving.
Prior to serving, sprinkle reserved coconut over the top.
Ambrosia salad looks best served the day it's made. The taste is not impacted if it's made up to a day ahead and served, but it won't look as good. The whipped cream will deflate.