Add the vegan sweetened condensed milk, soft brown sugar, and apple cider vinegar to a heavy-bottomed saucepan. Dissolve the sugar slowly over a low heat over 2-3 minutes, using a rubber spatula to stir it every so often.
Increase to heat to medium until the caramel comes to a simmer, continue to cook for 10 minutes, stirring frequently with your spatula to prevent the caramel from sticking to the base of the pan. DO NOT WHISK. The caramel will turn a deep amber color, then remove from the heat.
Slowly add the chunks of vegan butter to the caramel a few at a time, stirring between each one to combine. Once all of the butter has melted into the caramel, add the vanilla extract and sea salt and stir again. Lastly, stir in the soy milk until mixed through and you have a smooth caramel sauce.
Transfer the caramel sauce to a clean, dry 600ml jar, or two 290ml jars. Allow it to cool to room temperature for 15 minutes, before sealing the jars and placing them in the fridge.
Store in an airtight container or sealed jar in the fridge. It will thicken slightly once chilled, but it still holds a pourable consistency. It should keep for up to 2 weeks refrigerated in an airtight jar. Before serving, give the caramel a good stir with a spoon.
Keep this vegan caramel refrigerated. I've noticed that it has the tendency to form sugar crystals if kept at room temperature for too long.
Add the vegan butter slowly, a few chunks at a time. You want to bring the temperature of the caramel down slowly, not add all of the butter at once. Make sure you gently stir using a rubber spatula while the butter melts into the hot caramel.
Don't whisk the caramel sauce! The first couple of times that I tested this recipe I used a whisk to mix together the sugar with the other ingredients. I ended up with a nice caramel, that then started to crystalize once set.