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chakarera wrote

As far as I understand, with b12, methylcobalamin is better absorbed than cyanocobalamin but slightly more expensive, and injections work best followed by sublingual tablets, followed by chewable tablets. Though chewable cyanocobalamin works for most people (I had mild deficiency and it worked for me), but with a long term severe b12 deficiency people lose the ability to absorb or convert it or something and only respond to injections.

With vitamin D it's obviously preferable to spend time outside and get enough sunshine rather than supplement. But it requires anywhere from 15 minutes to a couple hours a day, depending on how much skin you have exposed, how dark your skin color is, and how sunny it is, to get enough. And not everyone can do that depending on their lifestyle. Now I am outside plenty and don't need to supplement but years ago I was working inside from before sunrise to after sunset and got zero sunlight except a little on weekends, and developed vitamin D deficiency that I fixed taking supplements. d3 is more effective than d2, but more expensive and also not normally vegan. Now there is vegan d3 produced by some kind of lichen. After the industrial revolution with babies being raised without sunlight rickets became a big problem until they started adding d2 supplement to milk. vitamin D deficiency isn't nearly as serious as b12 but can cause low energy/fatigue and bone health problems, I think it's recommended for vegans to supplement if they can't get enough sunlight.