Raise the oven's temperature to 170°C / 340°F.
In a small bowl stir together the Greek yogurt and orange juice until uniform. Set aside.
In a large mixing bowl add the eggs and beat lightly. Add the sugar and whisk to combine. Add the yogurt mixture and whisk until uniform.
Mix in the vanilla sugar, baking powder, and orange zest.
Gradually incorporate the oil while whisking. Once fully combined start to add the filo.
Take 2-3 handfuls of the filo and add to the mixture. Breaking the bigger pieces and separating any that have stuck together.
Start using a pastry spoon to stir the mixture. Pressing the filo to sink in the batter, keep adding handfuls of filo until you add it all.
Grease a 13 x 9 inch (33 x 23 cm) pan with cooking spray or vegetable oil. Add the batter and spread evenly.
Bake for about 30 to 35 minutes until deep golden lightly browned on top.
Ladle the cooled syrup over the hot pie.
Cut all the way through and divide into pieces to help the pie absorb the syrup. I cut it into 8 big pieces. You can divide the 8 pieces in half to get 16 smaller triangles or simply cut the pie into smaller square pieces to begin with. Though I doubt you'll have only one!
Allow the pie to sit at room temperature for 2-3 hours and absorb the syrup. It will look like a lot but it will absorb it all.
Once the orange pie gets to room temperature cover and refrigerate for at least 6-8 hours. Cold Portokalopita is the best but you can also enjoy it at room temperature if you prefer.
Serve with a scoop of Greek yogurt and cinnamon on top or see other suggestions in the text above. Enjoy!