I think the b-12 issue is exaggerated, to some extent.
The amount needed (sub-milligram) is so small, an occasional bug will do.
No, it's not really exaggerated, though it may be over-emphasized at times.
Although the amount needed is on the microgram level, it's still a highly critical nutrient, one which many vegans neglect.
The problem with B12 is that, unlike most other water-soluble vitamins, the body actually stores substantial amounts, so it can take years of deprivation before the deficiency is noticible. And by the time symptoms manifest, the damage is already being done, and may be irreversible if not caught soon enough. It's also difficult to diagnose, and is often misdiagnosed as other disorders; most frequently folic acid deficiency (treatment for folic acid deficiency can mask and exacerbate B12 deficiency).
So a vegan can manage for years without supplementing before deficiencies become a problem. This is probably the main reason why so many vegans revert to an ovo/lacto-vegetarian or omnivorous diet after a few years.
There is only one single vegetarian source for B12, but as noted before, many vegans are also prone to a substantial amount of dietary and medical woo which prevent them from supplementing their diet. Some eat a more restricted diet consisting only of "unprocessed" foods (raw food diet), which do not typically include nutritional yeast supplementation.
Because of the limited availability of vegetarian sources for B12, many pre-prepared vegan foods supplement it. However, this involves different issues; as many (most?) pre-prepared vegan and vegetarian foods include trans fats or naturally-occuring highly saturated fats such as coconut oil or palm kernel oil.