If you are thinking about B-12, that is already artificially added to meat products too. So even people who eat meat aren't getting it the "natural" way. Now there are available plant milks fortified with it which does the same thing.

Yes, vegans should monitor their health more closely to make sure nothing is missing, but it wouldn't be particularly difficult to get everything you need from plant based sources.

If modern medicine and things like vaccines are ok, then so are supplements.

Supplements are lower impact and less "unnatural" than animal agriculture.

I think that a reduced meat diet is good for the environment but being vegan is very far in my opinion.

I'm not a vegan, but that's not what they're saying at all.

They are comparing slavery and misogeny to the killing of animals, not the animals themselves.