Combine the vodka, frozen berries, simple syrup, fruit juice, and ice cubes in the blender.
Pulse the blender on low speed to start breaking down the ice and fruit, then increase to high and blend for about 30-45 seconds until the mixture is smooth and slushy, with a slightly icy texture.
Check the consistency; if it’s too thick, add a splash more fruit juice and blend briefly to incorporate; if too thin, add more ice and blend again.
Pour the icy slushie into glasses, ensuring each is filled with a vibrant, smooth frozen mixture.
Garnish with fresh fruit slices or a sprig of mint for a colorful finishing touch and serve immediately with a straw.