Back in the 80's early 90's if the words vegetarian or vegan turned up, immediately the question was what about protein. From then until the last few years, I never heard anyone express concern again about protein. Very recently I have overheard the same kind of conversation again. Curious. Protein is pretty much in everything. Protein must top the list of things that a vegan or vegetarian does not need to pay the slightest bit of attention to.
For vegans, but not so much lacto-vegetarians do need to keep this in mind. But the solution is easy. Beer!
If you look at B12 (and other B vitamin supplements) you will find that the ultimate source of B's is yeast extract which in turn comes from brewers yeast or used to. Decades ago as a project I looked at selling an "iso-tonic health drink" with b-vitamins. Basically, water, sugar, salt and yeast extract with a fancy name. The b-complex extract is pricey per kilo but a kilo of the stuff is an enormous quantity at the dilutions we need.