Peel, core and chop the apples and plums. Add to a large pot with water, vanilla, maple syrup & lemon juice.
Heat over medium high heat until lightly bubbling, reduce heat to medium low and cook until apples can be crushed by a fork, about 20 mins.
Use an immersion blender to blend sauce until smooth. If needed, add more water 1 tsp at a time to reach desired consistency. Seal in an airtight container and refrigerate or can.