You just need to put a little yogurt on it. (What were we talking about?)
...Nutritional Yeast is the only Vegan source of B12. But it's not necessarily even that great of one.
You can get your B12 from pills, and fortified Soymilk, etc. You don't have to risk life and death to use Nutritional Yeast.
It's microbial, and other than that I don't know. I really don't know what triggers a yeast allergy, and whether Nutritional Yeast would aggravate it.