Add dandelion petals to a saucepan with 1 cup water, 1/4 tsp vanilla, and two lemon slices. Bring to a boil, then cover, lower the temperature, and simmer for 30 minutes, stirring occasionally. You can add a little water (up to 1/2cup) if the liquid evaporates too quickly. Turn off heat, keep covered, and steep for 4-6 hours. The longer you steep, the stronger the flavor.