Preheat oven to 200°. Place a wire rack into a large sheet pan and set in oven. Your waffles will rest here as you finish cooking them.
Combine sugar and lemon zest in a large bowl and, using your fingers, scrinch them together, releasing the lemon oils into the sugar. Using a large whisk, combine flour, baking powder, baking soda, and salt with your lemon sugar. Set aside.
Melt butter and set aside to cool for a couple of minutes. In a large measuring cup, whisk buttermilk, lemon juice, oil, eggs, poppy seeds, and vanilla.
Add wet ingredients to dry and stir until just combined. Drizzle in butter while continuing to mix. Allow batter to rest for at least 10 minutes while you heat up your waffle maker. If you have a temperature dial on your waffle maker, set it to the medium setting.
Pour batter onto preheated waffle maker (amount suggested by your waffle maker). Cook to manufacturer instructions, or about 4-5 minutes.
Transfer the cooked waffles to the wire rack and keep warm in the preheated oven as you cook the rest. Repeat to cook the remaining batter.