Using a knife and cutting board, dice the seedless watermelon into roughly uniform cubes for easier blending.
Place the watermelon cubes into the blender, add the chopped mint leaves, and pour in the vinegar.
Secure the lid on the blender and blend on high speed until the mixture is completely smooth, about 1-2 minutes. Look for a vibrant pink liquid without chunks.
Stop the blender and taste the soup; add salt if desired. Blend briefly again to incorporate.
Pour the soup into a bowl or pitcher and refrigerate for at least 30 minutes to chill thoroughly. The soup should be cold and silky to the touch when ready to serve.
Serve the chilled watermelon soup garnished with additional mint leaves or a drizzle of vinegar for extra brightness, if desired.