- 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.