Is what you are making beer or sugar wash? Also your research didn't appear to find the minimum concentration of Vegemite needed to make the "beer", it found that 0.5% v/v of Vegemite to water was sufficient, with the next level down being no Vegemite which provides no yeast nutrients. Wouldn't it be expected that at a certain concentration of Vegemite between 0 and 0.5% v/v, only partial fermentation would occur, limited by the nutrient availability (similar to making a sweet mead) which would allow the minimum required concentration of Vegemite to be calculated?