I couldn't resist googling:
Vegemite in beer? - Home Brew Forums
Vegemite is made from autolyzed yeast. That is, yeast that have gone through their life cycle, sat around then "lysed" or broken open. The compounds in yeast, primarily the glutamates is what gives it the savory/meaty taste. In recent years, the flavor has been described as "umami". Obviously, Vegemite is dead yeast, so it won't ferment anything. At low levels, autolysis flavors offer some complexity in big beers. It is also a contributing flavor to traditional Champagne. Autolysis is generally a flavor we try to avoid in beer.
