In a medium saucepan over medium heat, combine the blueberries, water, and sweeteners. Bring to a boil.
Reduce the heat to a low simmer and cook 10 minutes. Remove from heat and use a fork to mash the berries a little.
Stir in the vanilla extract. Sprinkle the surface with the glucomannan, whisking briskly to combine. Let cool completely.
For a clear syrup, strain the mixture through a fine mesh sieve after step 2. Press on the solids to release as much juice as possible. Return the juice to the pan and stir in the vanilla, then whisk in the glucomannan.
Adjust sweetness to taste.