Shake the Vegemite in a jar with 1/2 cup of the liquid and the arrowroot. Once the remainder of the liquid is boiling, add this mixture.
Now, if you are creative and wanting to thicken a soup, use this idea. I tend to use just a small quantity of liquid, some arrowroot and Vegemite to flavor a thin liquid on all kinds of things because of the flavor, which is subtle, meaty and rich, when diluted.