How about Vitamin B12? This article:
https://en.wikipedia.org/wiki/Vitamin_B12
says: "There are no naturally-occurring notable vegetable dietary sources of the vitamin, so vegans and vegetarians are advised[5][6] to take a supplement or fortified foods."
There is also this part:
https://en.wikipedia.org/wiki/Vitamin_B12#Plants_and_algae
but who eats much algae, among vegetarians? Wonder if Spirulina is one of those algae. Need to check. Spirulina as a food supplement is commonly available in capsule form.
A lot of meat is also artificially enriched with B12 though and a lot of meat eaters are also deficient so a B12 supplement is probably a good idea for most people anyway.
A lil while googling about them seemed to say no-one's sure what good or bad effects they have, if any. And they're in a lot of things:
"According to one study...foods with the highest relative phytoestrogen content were nuts and oilseeds, followed by soy products, cereals and breads, legumes, meat products, ...vegetables, fruits, alcoholic, and nonalcoholic beverages."
And well, Japan has the world's greatest life expectancy doesn't it? And they have soy everything.
And before somebody jumps in to say Omega-3, not only can you easily get that from plants but fish also get it from algae. They don't produce it themselves.