Preheat oven to 350°F (175°C). Grease or line an 8x8-inch baking pan with parchment paper.
In a medium bowl, combine flour, oats, brown sugar, baking soda, and salt. Stir in melted butter until the mixture is crumbly.
Press ⅔ of the crumb mixture firmly into the bottom of the prepared pan to form the crust.
In another bowl, toss together diced rhubarb, mixed berries, granulated sugar, cornstarch, lemon juice, and vanilla extract until evenly coated.
Spread the fruit mixture evenly over the crust.
Sprinkle the remaining crumb mixture on top of the fruit layer.
Bake for 35–40 minutes, or until the top is golden and the fruit filling is bubbling.
Let cool completely before cutting into bars.