Whisk 1 cup sugar, butter, water, 1/4 cup lemon juice, and egg yolk together in a heavy-bottomed saucepan over medium heat. Bring to a boil, stirring constantly. Cook until thickened, about 1 minute. Cover sauce to keep warm.
Beat cream cheese, 1/4 cup mandarin orange juice, 1/4 cup sugar, 2 tablespoons lemon juice, and vanilla extract in a bowl using an electric mixer until well blended; mixture will be a bit lumpy.
Blend milk, flour, and eggs together in a bowl using an electric mixer. Set a greased crepe pan over medium heat. Ladle batter in until just covering the bottom of the pan. Cook until top dries, about 3 minutes. Flip and set aside; repeat with the remaining batter.
Spoon filling into crepes and fold over like a taco. Spoon mandarin orange pieces over filled crepes. Pour sauce on top.