Whisk together flour, sugar, and salt. Whisk together milk, vanilla, and eggs. Gradually add milk mixture to flour mixture, whisking until smooth. Whisk in melted butter. Strain through a fine sieve; discard lumps. Refrigerate batter for at least 2 hours or overnight.
Preheat oven to 200 degrees. Combine ricotta and zest. Lightly coat a 6-inch crepe pan or nonstick skillet with butter. Heat over medium heat until just starting to smoke. Remove from heat; pour 2 tablespoons batter into center. Swirl to cover bottom. Reduce heat to medium-low; return pan to heat. Cook until edges are golden and center is dry, about 45 seconds. Flip; cook for 45 seconds.
Slide crepe onto an ovenproof plate; cover with foil, and transfer to oven. Repeat with remaining batter, coating pan with butter as needed.
Fill each crepe with 2 to 3 tablespoons ricotta mixture. Fold into quarters to form a triangle. Make stacks of 3 crepes. Top with marmalade.