Sure, here you go: eating cobalt from the soil wouldn't give you B12, because bacterial fermentation in humans is occurring in the colon, while B12 is absorbed by the ileum.
But actually there's a solution for vegans. As the following paper in Nature eloquently puts it:
"Human faeces contain appreciable quantities of vitamin B12 or vitamin B12-like material presumably produced by bacteria in the colon, but this is unavailable to the non-coprophagic individual."