Preheat the oven to 350°F (175°C). Grease and flour a 9×5-inch loaf pan.
In a large mixing bowl, combine the yellow cake mix, sour cream, eggs, vegetable oil, 1/4 cup fresh lemon juice, lemon zest, and vanilla extract.
Using an electric mixer, beat the ingredients on medium speed until the batter is smooth and well combined, about 2 minutes.
Pour the batter evenly into the prepared loaf pan and smooth the top with a spatula.
Bake in the preheated oven for 45 to 50 minutes, or until a toothpick inserted into the center comes out clean.
Remove the loaf from the oven and allow it to cool in the pan for 10 minutes.
Transfer the loaf to a wire rack to cool completely.
In a small bowl, whisk together the powdered sugar and 2 tablespoons fresh lemon juice to make the glaze.
Once the loaf is completely cool, drizzle the lemon glaze evenly over the top.
Let the glaze set for 10 minutes before slicing and serving.