There are several different molecules that make up the B family of vitamins. The most predominant *external* source of B vitamins is animal origin food. Organ meats, muscle meat, milk, eggs. You’ll find similar materials in vegetarian foods, but they’re not the same.
The results from vegetarian sources are inconsistent and can’t be relied upon for optimal health.
Fermented foods are a good source of vitamin B complex, but each fermented foods comes with it’s own nutritional profile and getting a wide variety of true fermented foods, with live cultures in them, these days is difficult, unless you’re making those foods yourself. I make my own fermented foods, I suggest you do the same.
Kefir, kombucha, kimchi, sauerkraut, kvass are some examples of fermented foods that you can make on your own.
The internal sources
Gut bacteria can produce B vitamins for you, provided you have a healthy gut bacteria population. Each species will produce what it can, and they support each other in the gut with these vitamins. So, unless you have the entire ecosystem of microbes in place, working and supporting each other, internal B vitamin production will be sub optimal. The B vitamins produced by gut bacteria need to satisfy the demands of their neighbors first. What ever is remaining after their use of what is given to us humans (their host)
Supplements
Since there are several kinds of vitamin B, and all of them are important to human health, it is impossible to find the exact ratio suited for each individual. The requirement of each type will vary from person to person, based on activity, time of day, diet and health status. It is simply not possible to gauge the requirements accurately and provide it in the form of a pill. Some of the B vitamins will be excess in supply, and some deficient. The excess vitamins will cause certain processes to speed up and thereby consume other vitamins and minerals, causing deficiencies in other areas. Therefore, as a short term solution, capsules/supplements are very effective. They will lose their effect soon enough. And sometimes cause harm in the long run.
Solution
Our bodies have been managing vitamin B complex demand and supply very effectively for millenia. Give your body the appropriate raw materials, and the body will take off the rest. You just need to get out of it’s way. There’s no need to micromanage. A diet that supplies animal food is the easiest method. A non vegetarian or lacto vegetarian diet that varies it’s sources regularly, and according to season, instead of sticking to the same food types daily is your best bet.
A vegan diet is a bit of a gamble. It might work, it might not. Supplements should be used as targeted short term corrections. Long term use will backfire in unpredictable ways.