We don't need much B12. Somewhere in the 2 - 12 mcg range. So you might think that 100 is more than enough.
However, the absorption site easily gets overwhelmed. I don't have the numbers memorized but after the first 1 mcg, the absorption site stops working for a few hours. So after that, the efficiency of absorption drops down to 1%. If you do the math, it turns out you need 1000 mcg.
There is another alternative. you can take a smaller amount, like 50% of the RDA, two or three times a day A lot of the B12 fortified foods have that amount. So you can just get some b12 fortified plant milk and have 2 - 3 glasses a day (maybe one glass with each meal).
There are some other products on the market that claim to bypass the absorption issue. There are sublingual drops, B12 fortified toothpaste, etc. The concept is that the B12 goes right into your bloodstream. I don't know if they really work.
There is also a timed-release B12 supplement. but again I don't know if it works.
You might try a few of these. B12 can be stored in the body and even large dosages are harmless.
I have at least one or two glasses of soy milk a day but I don't think that might be enough so I also take 1000 mcg that is time-released. At 7¢ a day its cheap insurance.