Put the sugar, cornstarch and salt in a medium size saucepan and mix together well.
Stir in the water, cranberries and raisins.
Put the pan over medium heat and bring to a boil while stirring constantly with a wooden spoon.
Reduce the heat and simmer for five minutes, stirring constantly so the mixture does not stick.
Remove the pan from the heat and set on a heatproof surface.
Carefully stir in the vanilla, and set the pan aside to cool to lukewarm.
Meanwhile, preheat the oven to 350°F (175°C).
Grease an 8 inch (20 cm) square pan that is about 2 inches (5 cm) deep with the extra butter or some plant-based butter.
Combine the rolled oats, brown sugar and flour in a medium size mixing bowl.
Add the butter or dairy-free spread and combine the ingredients, using a pastry cutter or a fork until the mixture is crumbly.
Sprinkle half the mixture into the prepared pan and press it down gently with your fingers.
Carefully spread the lukewarm cranberry mixture over the top.
Sprinkle the remaining crumb mixture over top of the cranberry mixture.
Bake for 30 to 35 minutes or until the topping is beginning to turn a light golden brown.
Remove from the oven. If serving as a dessert with ice cream, the Cranberry Oat Crumble Bars can be served warm, however be careful to allow them to cool a bit so they are easy to serve and nobody burns their mouth on hot cranberries! If cutting the crumble into smaller bars, it's best to let the Cranberry Oat Crumble Bars cool completely first.