Yes, Marmite is vegan.
Yeast is a type of microfungi and as such is not considered non-vegan or an animal product, just like any other kind of mushroom.
Regarding yeast and veganism, there are already some answers in this thread.
Marmite is vegan. However, if you are vegan because of the animal rights side of things, you should take into consideration that it is made by Unilever and they are heavily involved in animal testing. Most of the main supermarkets do a generic brand, all of which taste no different.
Nutritional yeast is best consumed in small amounts daily.
Nutritional yeast is a very high source of niacin (Vitamin B3). The tolerable upper limit (TUL) for niacin is set at 30 mg per day because some people experience uncomfortable facial flushing after consuming anywhere from 30-1000 mg of niacin per day. One tablespoon of nutritional yeast provides 28 ...
The term nooch as a nickname for nutritional yeast seems to have originated at the Post Punk Kitchen forum way back. I say seems because I was not able to find any absolutely trustworthy source for this, but I did find a couple of weak ones (Urban Dictionary entry for nooch; this site and also this Post Punk Kitchen forum post that wonders about people ...
Water, yeast, and salt are all vegan ingredients.
Flour could be non-vegan if it was bleached by an animal product (usually bone chars). This excludes every non-white flours, every white flours that were not bleached, and every white flours that were bleached by an inorganic chemical oxidant, such as potassium bromate.
So, answering your question: if you ...
Yeast, like mushrooms, are eukaryotic micro-organisms, which are part of the fungi kingdom, so yeast is not biologically classified as an animal and is thus vegan-friendly.
There are of course other things in bread that may be non-vegan such as butter and milk.