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Unfortunately for us, lots of cereals are fortified also with D3, which is usually derived from animal products and knocks a lot of Kellogg's offerings on the head. Sorry bub, just eat some oats, it's far better for you than a bowl of Crunchy Nut.
As for B12, we don't get it from well water or living with our livestock any more, so we're all dealing with the same deficiencies in our food chains, in that livestock under factory farming conditions are supplemented with it to replace its loss due to, well, factory farming. You're either supplementing the cattle, or supplementing yourself. I'd recommend the Vegan Society's multivit, it's cheap as chips and the 180 tub will last you half a year. If you really want to go deep on B12, have fun reading this entire fucking Wiki full of citations and evidence-based approach from a fairly respectable source (although aggravatingly it now seems they DO have a popup advertising some kind of "pay us to tell you how to be vegan" on their site, but the information is still all free): https://veganhealth.org/vitamin-b12/
Approached sensibly a vegan diet can be just as healthy (if not usually more so, unless you live off frozen vegan meat substitutes, chips and Lotus biscuits*) than an omnivore/carnist one. Usually people end up losing weight and getting better skin because, up until about 5 years ago, you were fucking forced to cook for yourself. Which is good, because it teaches people to not be scared of their own kitchens and think a bit more about what goes in their gobs.
*Look fuck you, buddy, just because that's all I've eaten today doesn't make me a bad vegan or unqualified to give this advice, it just means Lotus biscuits are the fucking tits and I'm on my period. Which, incidentally, doesn't even affect my haem levels any more. I'm not about to say going vegan fixed my anaemia, but I couldn't donate blood when I was omnivore and now I can, so shut up and go make an appointment at your local donor centre, thank you.
Luckily for you, times have been a-changing and now most chain restaurants are having to cater to a vegan section of the market. Unless you live in bumfuck nowhere, you can find somewhere with vegan options eating out in nearly all urban areas. Even fucking Ember Inns has a vegan main now (lentil and sweet potato cottage pie, if you were wondering, and it's alright) so you don't have to miss any awkward Sunday meals with your da.
Honestly, the most stressful thing about being vegan is dealing with other people who become overnight nutritionists as soon as they hear the V-word. But we already know hell is other people, and you must be a cantankerous cunt if you're posting here, so I'm sure you'll cope.