Preheat the oven to 350° F.
Add vinegar to plant milk and set aside. This is your "buttermilk."
Using a standmixer (whisk attachment) or handmixer, mix together butter and sugar until the butter is creamy. Add aquafaba (or plant milk), vanilla extract, and buttermilk and mix until ingredients are combined. It'll look curdled, but don't worry.
Whisk together flour, baking powder, baking soda, and salt. Pour the dry ingredients into the wet ingredients and mix until just combined (do not overmix). The batter will be very thick--much more like a biscuit batter than a cake batter. This is what you want, but if it's simply too thick to work with, add 1 tbsp of plant milk to slacken it a bit.
Scoop out approximately 1/3 of the batter into a separate bowl. Add 1 cup of the thawed berries (together with any juice) to this smaller portion of the batter. Mix the berries into the batter, until it turns a lovely bright purple color.
Add the remaining berries to the larger portion of the batter, using a spatula, so as not to break the berries.
To a 1 lb loaf pan, generously misted with cooking spray, Add a layer of the light colored batter. To that, add two generous scoops of the purple batter. Then repeat until all the batter is used up.
Using a large wooden skewer or a chopstick, swirl the batter around a few times.
Place in the oven and bake for approximately 50-55 minutes, until toothpick comes out clean.