Combine the rhubarb in a bowl with 3/4 cup of sugar, and let sit for 15 minutes
In another bowl, mix the strawberries with 1/2 cup of sugar and let stand for 10 minutes, stirring occasionally
Using a slotted spoon, add the rhubarb to the strawberries, being sure not to add any liquid from the rhubarb bowl. (You can save this rhubarb syrup for another use)
Add the cornstarch, lemon juice, and vanilla to the fruit and stir well.
Transfer the mixture to a 9 x 13 inch glass baking dish.
In a large bowl, add all the ingredients for the topping (butter, light brown sugar, all-purpose flour, rolled oats, sunflower oil, cinnamon, and salt), and then mix with your hands until large crumbs form.
Sprinkle the topping evenly over the filling and bake for 30 minutes. Reduce the temperature to 325 and bake ~30 minutes longer, until the fruit is bubbling and the top is nicely browned.
Let the crisp rest for 10-20 minutes before serving.