I agree with everybody on this thread.
I have something to add to this discussion but I am also going to repeat some of the stuff that has been said. Think of it as emphasis.
the best source of nutrients is food. Supplements are ok but not nearly as good as food. Besides all the micronutrients that food contains there all kinds of phytonutrients that we require. And they aren't in the typical supplement. Heck, we don't even know all the name and roles of all these phytonutrients.
B12 is not in any plant foods. But it is added to some of our foods. All the plant milks have B12 added. Two or three glasses of soy milk will meet your B12 requirements. So will an inexpensive multivitamin. If you buy a B12 supplement, many of them come in "megadoses". So, for instance, if you buy one that is 1000% of the RDA you probably can just take it once a week. I saw one recently that was 10,000% of the RDA. You could take that one once a month or less. B12 is easily stored in our bodies. some people don't absorb B12 that well. you don't need a megadose unless a blood test says you are deficient.
Best bet is when you get your annual check-up and blood test your doctor will tell you if you have any deficiencies. Then act accordingly. Perhaps add some more and different foods to your daily routine. or take a supplement. Even though our bodies make Vitamin D, many people (even nonvegans) are deficient in it. We just don't get outside enough. And when we do we wear clothes and hats or sunscreen. Plant milks have added D and calcium.
Even if you ingest all the recommended amounts doesn't mean that you absorb it all. Non Heme iron is not as absorbable as Heme Iron (Heme iron is only found in animal products). Some foods inhibit the absorption of other nutrients.
If your diet isn't great - or great every day, a multi is just a very cheap insurance. I take a vegan multi called DEVA tiny tablets. It costs about 6¢ a day.
the only other pill I take now is an Omega 3. Although I add a few teaspoons of ground flax seed to my food every day, I take a vegan Omega 3 to make sure I get enough. I recently switched to Purity Products Omega 3 - 6 - 9. It's about 25¢ a day. I'm also a big fan of soy milk.