Place 8 ounces trimmed and chopped rhubarb in a small, heavy-bottomed sauce pan with enough water just to cover the rhubarb. Scrape the vanilla seeds from the 1/2 vanilla bean pod into the mixture, before throwing the pod into the mixture as well. Bring to a boil, then simmer for about 20 minutes until the rhubarb is soft and you have a pink juice. Strain out rhubarb pieces and vanilla pod and let cool. Measure out 2/3 cup of juice for the curd.
In another medium, heavy-bottomed sauce pan, whisk together 2/3 cup rhubarb juice (from the 1st step), 2/3 cup sugar, and 4 large eggs. Cook over medium-low heat, stirring constantly and scraping the sides and bottom of the pot with a wooden spoon, just until the internal temperature reaches 170 (F). If you don't have a thermometer, watch for the mixture to thicken until it is able to coat the back of the spoon.
Once the mixture has reached 170 (F) and coats the back of a wooden spoon, remove from heat and whisk in 1/2 cup unsalted, cubed butter a few pieces at a time, only adding the next cubes when the cubes before have melted. Once all the butter has been added, strain through a fine mesh sieve and allow to cool to room temperature.
Once the curd has cooled to room temperature, cover and refrigerate for at least 2 hours until firm. The rhubarb will keep in an airtight container in the fridge for up to 2 weeks.