Don't give up on injecting. You focused on one of the ingredients of Fab-B.
Forget about that one component and look at the rest.
FAB B Contains: Hydrolyzed soy protein, vegetable oil (soybean and or corn, cottonseed), sodium phosphates, mono sodium glutamate, autolyzed yeast extract, disodium inosinate and guanylate, xanthan gum.
Skip the soy protein and phosphates. The xanthan gum is a thickener, so out it goes. That leaves the MSG, Oil, the yeast extract and the two disodium chemicals. The two disodium chemicals are flavor enhancers and likely hard to get, so out they go.
The yeast extract is rich stuff. Check your Vegemite, I suspect that it is mainly autolyzed yeast extract. A little goes a long way, just like the MSG.
We have two ingredients, now what about the oil. I'm not sure the MSG is oil soluble, so it likely needs to be dissolved in the Vegemite. The sodium phosphates is an emulsifier to keep the oil from separating from the water soluble ingredients. I think this is starting to come together.
We could dissolve the MSG in the Vegemite and then whisk in oil, injecting before it has a chance to separate. We could also dissolve everything in water or wine, or bullion and forget about the oil.
If it were me, I would go with a strong bullion, like twice as strong as recommended. I can't be cheap bullion cubes. It needs to be a quality beef base or good bullion like Knorr. Write down the proportions as you go.
Start there and add a tiny amount of Vegemite. Once that tastes good, (stop before it overpowers) add a pinch of MSG. It is easy to over-do the MSG.
Sounds like a plan to me. Vegemite is hard to come by here in the Texas Tropics, I'll check Amazon and see if I can get some. I'm in.