Start by preheating your oven to 350 degrees and greasing an 8x8 pan or place foil with an overlap so you can pull the bars out in one piece, grease the foil.
If you are using frozen berries, unthaw them in a medium bowl; it's okay if some are still frozen; you don't want them complete mush. Add the remaining filling ingredients and stir gently and set aside.
Now start the crust. Take a medium-size bowl and add the flour and sugars; stir. Take the vegan butter and a pastry cutter (or fork) and crumble the butter into the flour mixture until it's combined well, it will be a bit crumbly, and that's okay. Add your vegan milk and fold the batter until it absorbs the dough. This is a thick, somewhat crumbly dough (kind of like making biscuits).
Take the dough and press it evenly into the bottom of the 8x8 pan. Bake this for 9 minutes.
Remove from the oven and place the graham crackers on the top of the dough, covering the majority of the crust. Now take the berries they will be liquidy and pour them over the crust.
Bake this for 40-45 minutes. The berries will thicken as cooked.
Wait a few hours before cutting, or it will be too hard to move. You can speed this up by placing them in the freezer for 30 minutes and then cutting. These taste great cold from the fridge.