Glad to see you're considering a vegan diet/lifestyle and of course, I encourage that for a whole host of reasons.
I just completed a plant-based nutrition certification course through Cornell University and hopefully I'll be able to answer some questions. There are plenty of other resources out there as well.
As far as the B-12 question, Vitamin B-12 doesn't come from animals or plants. It is a bacteria. Because our food and water are so sterile these days, or as the article referenced, "improved sanitation," Vitamin B-12-containing bacteria is hard to find, other than in animal products, which tend to contain lots of bacteria. Therefore, these days, a Vitamin B-12 supplement is a good idea and is generally advised for those not eating bacteria on their food. This has nothing to do with whether we're "meant" to eat a certain way, if we have to supplement, what it means is that our culture has changed, soil has changed, the way we grow, harvest and produce food has changed, and the only Vitamin B-12 bacteria that is readily available happens to be found in certain types of meat.
The fact that Vitamin B-12 is found in certain types of meat doesn't offset all the reasons why meat and other animal products should be eliminated from the diet. There are plenty of studies to reference, plenty of books, and plenty of observations one can make within one's community of the health associated with those who follow a standard western diet, and those who adhere to a plant-based diet. The difference is usually quiet profound. Likewise, profound differences are seen when one rejects an animal based diet after living that way for a long time, perhaps a lifetime, and adopts a plant-based diet. During my work with the film Forks Over Knives we were able to see thousands of first hand testimonials from those who switched to a plant-based diet and had health improve dramatically.
For some additional resources or research you want to do, I suggest some of the following:
Dr. John McDougall
Dr. T. Colin Campbell
Dr. Caldwell Esselstyn
Dr. Doug Lisle
Dr. Pam Popper
Dr. Michael Greger
Dr. Michael Klapper
Dr. Dean Ornish
Dr. Joel Furman
And plenty of others.
A fun website run by a doctor who reads every scientific paper in the English language about health and disease each year is Dr. Greger's website http://www.nutritionfacts.org
All the very best!