Add frozen berries, water, apple cider vinegar, and erythritol to a small pot over the stove and heat with a lid on until the berries thaw and break down. When the berries are soft enough to mash with the back of a fork, you can remove the pot from the stove and use an immersion hand blender to puree the berries or puree the berries in a food processor. After pureeing the berries, push them through a sieve to remove the pulp and seeds.
Pour the berry puree onto a dish and set it in your refrigerator to thicken and chill. Slice a lemon in half and squeeze all the juice from both halves into a glass, then pour 8-12 ounces of water into the lemon juice, depending on how strong you like your lemonade. Stir liquid stevia and a pinch of pink Himalayan salt into the lemonade, then taste and adjust the sweetness and salt to your liking.
To assemble the lemonade, pour the chilled berry puree into the base of a 12-ounce glass. Fill it with ice, being careful not to splatter the puree. Slowly pour your lemonade into the glass, letting the berry puree infuse the drink. You may also choose to stir the lemonade and berry puree together into one purple drink.