Combine the water, sugar, orange juice, orange zest, and vanilla bean paste in a small saucepan. Cook over low heat until the mixture comes to a boil and sugar has dissolved.
Let boil for 1 minute, then remove from heat and let cool completely.
Add the buttermilk and stir to combine. Place mixture in the refrigerator, covered, for several hours or overnight to chill through.
Churn in an ice-cream maker according to manufacturer’s instructions. Store covered in freezer.