Put some of the 500 ml soy milk in a separate container and mix with the cornstarch and sugar. Scrape out the pulp of the vanilla pod.
Bring the rest of the soy milk to the boil in a saucepan with the vanilla pulp. When the milk boils, add the mixed cornstarch and immediately stir vigorously with a whisk.
The whole thing now has to boil again and thicken properly. That happens relatively quickly. Let the pudding cool and serve.