Measure the chilled lemonade and pour it into the blender.
Add the crushed ice to the blender, creating a mixture that looks frosty and slightly slushy.
Drop in the vanilla extract if using, which will add a subtle sweetness and aroma to the slushie.
If you want a fruity twist, pour in the berry juice for a vibrant color and flavor boost.
Secure the lid tightly on the blender and blend on high speed until the mixture becomes thick, smooth, and has a fluffy, icy consistency, about 30-45 seconds.
Stop the blender and check the texture; it should be scoopable and crackly when you insert a spoon.
Pour the slushie into glasses, allowing the icy, colorful mixture to overflow slightly for a tempting presentation.
If desired, garnish with a slice of lemon or a sprig of mint for a fresh touch.
Serve immediately with a straw or spoon, enjoying the refreshing, icy treat.