Combine rhubarb, sugar, and vanilla sugar in a pot and marinate for 30 minutes. Stir in apple juice and bring to a boil over low heat. Simmer until rhubarb is soft, 4 to 5 minutes.
Stir cornstarch with a little bit of water and stir into the rhubarb. Bring to a boil and remove from heat. Cool completely. Stir in strawberries before serving.
