Prepare the oranges: In a large bowl, combine 3 parts water with one part white vinegar and let the oranges soak in there for 10 minutes.If you want to include orange peel in the curd, peel 2 of the oranges and finely chop the peel. Set aside.Juice the oranges, and pass the juice through a fine-mesh sieve to remove any seeds.
Add the soy milk, sugar, and cornstarch to a saucepan and whisk until smooth without any lumps. Add the orange juice, orange zest (if using), and lemon juice, and whisk again to combine.
Place the saucepan on medium-high heat and bring to a simmer. Turn down the heat slightly to a gentle simmer. Cook for 12-15 minutes, while stirring alternating with a rubber spatula and a whisk to prevent the mixture from sticking to the pan. The mixture will thicken and should coat the back of the spatula.
Remove the saucepan from the heat and stir in the vegan butter, vanilla, and salt until completely incorporated and you have a smooth curd. Allow it to cool to room temperature for 20 minutes.
Storage: Transfer the curd to an airtight jar and refrigerate for up to a week. The curd will thicken in the fridge, so you can gently heat and melt it in a saucepan anytime you need a runnier curd consistency.
