Commercial medical inventions claim that Vitamin B12 is supplied only by non-vegetarian food like meat, fish, egg. It is purely a biased claim. But no where non-vegetarian can be food for human body. So vegetarians are pressurised and provoked towards non-veg. Don't worry, practices in Indian tradition based on ancient Ayurveda has accurate solution for this.
Indian winter months having chilling weather are Saha and Sahasya (tolerable, November-December-January). During this time the Vitamin B12 deficiency gets aggrevated. Sages identified this and provided remedy in Makara Sankranti celebrations. Solution is in our tradition all over India in one or the other form.
This is the Suggi (ಸುಗ್ಗಿ) or harvest festival for farmers of Karnataka. On this auspicious day, girls wear new clothes to visit near and dear ones with a Sankranti offering in a plate and exchange the same with other families. This ritual is called "Ellu Birodhu." Here the plate would normally contain "Ellu" (white sesame seeds) mixed with fried groundnuts, neatly cut dry coconut and fine cut bella (jaggery). The mixture is called "Ellu-Bella" (ಎಳ್ಳು ಬೆಲ್ಲ). The plate contains shaped sugar candy moulds (Sakkare Acchu, ಸಕ್ಕರೆ ಅಚ್ಚು) with a piece of sugarcane. There is a saying in Kannada "ellu bella thindu olle maathadi" that translates to 'eat the mixture of sesame seeds and jaggery and speak only good.'
Astonishingly this traditional "Ellu-Bella" mixture is the solution for Vitamin B12 deficiency. Why didn't researchers found this in Vegetarian diet? Because Vitamin B12 is not directly available in core form. Even in creatures whose meat is considered as source of Vitamin B12 it is synthesised in their body. Empherical analysis of that kind of meat resulted in Illusion that Vitamin B12 is available only in non-veg.
But eating the Ellu-Bella mixture, Vitamin B12 gets synthesised in human body. So they made it a ritual to eat one handful of this mixture at least once an year proper astronomical event. Proper Ayurvedic based Scientific experiments can prove this. Brahmarshi K S Nityananda Swamiji, Veda-Vijnana Mandira, Chikmagalur, authentically claims these sayings of sages who propounded original Ayurveda from Vedas.
There are separate names for vitamins, minerals, etc., in Vedas. All these comes in Vaidhika Jeeva Shaastra. Actually Vitamin B12 increases concentration power of the brain. During death rituals also "Tilodaka" (Sesame+water) is given to separate the mind and soul attachment through brain. All these are higher level lessons in Ayurveda.
So every ancient Indian traditional practice has scientific basis. Exploration without prejudiced and non-commercial mindset is the key to success in real medical science, the Ayurveda.