Reduce the oven temperature to 300 degrees F.
Combine the eggs, egg yolks, sugar and flour in a saucepan over low heat and whisk constantly to combine. Whisk until the sugar has dissolved, then whisk in the zest and juices of both the grapefruit and lemon.
Increase the heat to medium-low and continue to whisk constantly until the curd thickens and bubbles appear on the edges, about 6 to 8 minutes. Remove from the heat and whisk in the vanilla extract and salt.
Cut the softened butter into pieces and whisk it into the curd, one piece at a time. Whisk each piece until it fully melts. When all the butter is in, pour the curd into the crust.
Bake the bars until the curd is set and the center is slightly jiggly, about 25 to 30 minutes. If the curd still isn’t set after 30 minutes, bake it at 5 minute increments, checking after each time. You want only the center to be slightly jiggly. Remove the bars and let cool completely.
Once the bars have cooled, refrigerate them for at least 4 hours or even overnight. To slice, lift the parchment paper edges to remove the bars in one full square. Slice the bars into 12 or 16 pieces. Sprinkle with the flaked sea salt and serve.