I think Vegemite doesn't have a Best Before date because really there is no best. They make it. It doesn't taste good. And that's as good as it gets. Maybe it should have a "Worse After" date? But I suspect it's one of those things that doesn't ever actually "go bad", like gravel.
One way to bring out the flavor:
It's like homeopathy where the more dilute, the stronger the result