Pre-heat oven to 350 degrees. Spray a 9x5 loaf pan with the vegetable oil making sure to cover the sides and bottom well.
Whisk together the flour, poppy seeds (if you're using them), baking powder, baking soda, and salt all in a medium sized bowl.
Then in a large bowl, mix together the sugar, milk, eggs, oil, lemon zest, lemon juice and vanilla until well blended.
You will then add the dry ingredients into the liquid ingredients and stir until combined, but don't over mix.
Scrape the batter into the prepared loaf pan and bake for about an hour, or until a toothpick comes out clean. Let the pan cool for 10-15 minutes, then remove from pan to completely cool.
For the lemon frosting, sift the powdered sugar into a medium bowl, then add the lemon zest and stir. Mix in 1 tbsp of water, as well as the lemon juice in order to make it a pourable icing.
Pour the frosting over the loaf, either when it is still a little warm or at room temperature.