You did ask for reasons why one would not choose Vegemite vs this so felt obligated to give you the reasons i could think of HAHAHA:)
You left out taste. 
Vegemite is made by Kraft, which is a huge manufacturer of food products, hundreds if not thousands of them. If they were poisoning people we would all hear about it, it would be huge news. Is this proof? Maybe not absolute. I bet they follow accepted standards of manufacturing, though.
But what do you know about the guy packaging up your powder? Are his containers clean and sterile? What about his utensils? His work area? Is he wearing clean clothing? How do you know he didn't sneeze on it or drop some on the floor? Does he even wash his hands first? You have no idea.
Call me crazy, but I would feel safer eating something from Kraft, than powder out of a baggie I got on ebay.
Vegemite is made from beer tailings. Epicor is made is a very different environment with a lot more grain and a lot less water. Probably evolved into quite a different strain of yeast, creating a very different mix of metabolites. I'd want to see the studies showing the same immune-boosting effects before I assumed that Vegemite and Epicor had any similarity.
That and Vegemite is some amazingly
nasty tasting stuff. I've got family in Australia and they tried to serve it to me on multiple occasions. Never could stomach it. Diamond V XPC isn't great tasting, but it's not as bad as Vegemite.
Finally, you have quite a bit of trust in big food companies. Gemeral Mills, Cargill and ADM have spent a lot of money making sure that breakfast cereal (and everything else grain-based) is marked "heart healthy" by the American Heart Association when processed grains are the perfect recipe for high triglycerides, high blood sugar, high insulin, atherosclerosis, and an nearly guaranteed heart attack (the very opposite of heart healthy). Or that those same companies wouldn't manipulate the American Diabetic Association to never recommend low carb diets (despite the fact that diabetes is a failure of carb metabolism). Those three companies are the biggest contributors to both the AHA and the ADA.
You'll have to pardon me when I say I'm a little skeptical about the good intentions of big agribusiness. Not that Vegemite is bad for you (I suspect it's great for you), but you seem to assume that they wouldn't cover up an embarrassing event to protect their business. I think that assumption is mistaken.