Preheat the oven to 200 ° C top / bottom heat or 180 ° C convection.
Melt the margarine in a larger saucepan. Mix the flour, sugar, oatmeal, vanilla sugar and salt in a bowl. Stir the flour mixture into the melted margarine with a fork to form the crumble.
Place baking paper on a baking tray and spread about 2/3 of the crumble on it and press it down. Place the pitted plums on top and spread the remaining crumble on top of the plums.
Bake the cake in the preheated oven for about 30 - 40 minutes until golden brown