You can pick one of the usual Native American crop patterns and get a solid set of vitamins. Potatoes + beans + squash or something like that, maybe with some corn. Cf. Mann's 1491. If you're going for a minimal veggie ingredient diet these new-world combos work well as a base, in part because potatoes are pretty much a superfood.

B12 is a pretty important one if you want to avoid the title of section 1 of this article though.

And while butter contains b12, you'd probably have to be eating a few bars of it a day to get enough long-term.

That's my understanding.

The spudfit guy did only potatoes and a B12 supplement for a year. His claim was that he was getting everything else he needed from the potatoes.

I did the SMTM study and they prohibited dairy. So I got my B12 from sweet potato.