Pour one inch of water into a medium pot that accommodates your heat-proof glass bowl without letting it touch the water's surface. Bring water to a simmer over medium heat.

In a food processor or mortar and pestle, combine the sugar, grapefruit zest, and vanilla seeds, and pulse or mash until well combined and very fragrant.

Combine the grapefruit sugar, eggs, and egg yolks in the heatproof bowl.

Whisk together for one minute to distribute the sugar. Place the bowl over the pot with simmering water and whisk constantly until the sugar is dissolved, about 30 seconds.

Add the grapefruit juice and lemon juice and cook, whisking frequently, until the curd reads 170° F and has the consistency of sour cream, about 15 minutes. Remove the bowl from the heat.

Whisk in the cold butter little by little, waiting until each addition is completely incorporated before adding more.

Strain the curd through a fine-mesh sieve into a clean bowl.

Chill completely before using.