I was in the states for a month earlier in the year and only found one American who didn't like Vegemite when prepared properly. That guy's tastes were very much towards kids food. He ate a lot of nuggets, chicken strips, hot dogs and other crap.
Everyone else was sneering before they tasted it, expecting it to be ghastly. They were pretty surprised that they liked it.
Try it on toast with butter or margarine and apply about a 1/4 teaspoon of Vegemite, making sure it mixes through the butter.
That's a proper first time Vegemite experience. It may look like Nutella (a bit), but it is not. If you apply it the same way, of course it will be horrible, likewise don't try it on it's own or without butter.