the answer is "depends".
Since your main concern is B12, let's just talk about that.
The amount your body needs is literally microscopic. And for vegans, there are no food sources that contain it. But all B12 is initially made by bacteria. A cow gets B12 from the bacteria in its gut. A lion gets B12 from eating the cow. A vegan gets B12 by eating a vitamin. The Vitamin gets it from a vat of bacteria.
The form of B12 in the vitamin is different from the form made in a cow's gut. But it is still B12 and some research suggests that is just as if not more "absorbable" than other forms.
A lot of the people (nonvegans in particular) who are deficient in B12 aren't deficient because they don't eat enough (remember the quantity is super small). but because they can't absorb it that well or maybe even more important - can't utilize it that efficiently.
Elderly and sick people fall into that category.
This is why some B12 supplements have huge amounts - like 1000 to 10000 percent of the RDA. The idea is that if you take a lot of it, may be just a little will get absorbed and utilized. And all you need is a little.
If absorbing B12 is the issue there are a few things that can be done to help. One thing is that its thought that taking small doses regularly with food is better than to take large doses irregularly. So maybe the best thing is fortified foods. A couple of glasses of fortified plant milk plus a supplement could be the best bet.
But if you are healthy, there should be no issue with taking a mega dose once a week or less. B12 is very easily stored. Most people have enough about a 3 month supply in their liver.
If you somehow felt compelled to get it "naturally", a bowl of clam chowder once a month should do it. Or a 3 oz slice of cow's liver would last you 3 months.
But if the tablets don't work you should see a doctor. the fish won't work either.