Naturally, the thread title should be "Are vegans allowed to eat yeast?", as it removes the temptation of making a joke about vegans being so anemic they can't overpower teeny little beastly yeasties.
Ahem.
The way I see it, for people who are vegans because they don't like to have animals suffer/die for their meals, there is no right answer here. I mean, if the objective is not to exploit, then eating yeast should be fine. They don't suffer, they can't suffer, and really, cooking using yeast would be like paradise for them - warm and plenty of sugar. It is their ideal environment, right up until they get eaten.
Of course, yeast are technically fungi. But we are talking about eukaryotic micro organisms - the distinction between plant and animal on this scale seems really arbitrary. If it helps, assume I am speaking hypothetically about a non-plant micro organism - could vegans eat these?
OTOH, if you agree that micro organisms can't suffer and can be eaten, then you have made a judgement call. You have drawn a line in the sand and said these guys are fine to eat. But what about chickens? Chickens are so stupid that even if they can suffer, I doubtt hat they are even aware of their suffering. If chickens are too highly evolved, what about insects? They tend not to have nervous systems capable of transmitting pain and are a rich source of protein.
Yeast. So cool.