Preheat the oven to 350F.
Combine the sugar and cornstarch in a large bowl, then toss with the plum halves and vanilla.
Pour into a 11" round casserole dish.
Combine the remaining ingredients and mix until a crumble is formed. Top the plums with the crumble mixture.
Bake at 350F until topping is golden brown and fruit is bubbling at the edges (about 1 hour).