Just to clarify, I am from the UK and so I am talking about UK Marmite:
As you can see from the pot, they say it's vegetarian, but it's yeast extract, so does that mean it's not vegan? Does yeast count as an animal product?
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.
Marmite, along with a number of other plant based food products, are owned by Unilever, one of the biggest animal testing companies in the world. To buy their products is to condone the torture and murder of animals experimented on and therefore Marmite is not Vegan. Veganism isn't a diet, it's to live your life not exploiting animals in any capacity as far as that is practically possible.
Other yeast extracts are available and are better value.