No its not! (Sorry for shouting!) It's better and is the original! Sheesh, that's the trouble with our British ex-colonies, they take our ideas and change them and then try to claim they invented something!
Vegemite is a little milder in taste than Marmite. It's a love or hate foodstuff. My family LOVE it! I often have Vegemite in the house, though, cos my sister lives in Oz and she and her children and their partners and their partners sisters, brothers, inlaws etc descend on me at regular intervals!