Inositol, which is classified as a member of the B-Vitamin complex, is often used to cut cocaine *hydrochloride* due to having similar physical properties, as well as inert taste/smell. This isn't the same as B-12, and it isn't even considered a Vitamin since it can be synthesized within the human body, but you can buy it at health food stores.
I honestly have no idea what is used to cut crack, because normal cocaine HCl cuts, like lactose, mannitol, inositol, etc., are water-soluble, and would (i'm assuming here) stay in solution during a "cook," while cocaine freebase precipitated out. So basically, your guess is as good as mine.