Add the coconut milk to a saucepan. Add the sugar, and bring to a boil, stirring to dissolve the sugar. Simmer on medium-low heat for around 25 minutes, stirring now and then. You're done when the mixture is very thick and a bit gloopy, which is usually when the mixture's been reduced to around 3/4 of a cup.
Remove the pan from heat
Whisk in the vanilla and salt. Let cool for around 10 minutes.
Whisk in the soy milk, stirring vigorously, until very smooth. Let cool.
Store for up to 1 week, in an airtight container, in the fridge. The mixture will thicken a bit as it chills.