What makes you test positive for vicodin if you didn't take it?

Hard to say: If only a screening test was done, without confirmation, no one can say with certainty that is was hydrocodone. If confirmed, and there was no chain-of-custody there is always a small possibility that the specimens were mixed up, either at their source or in the lab. It can also be a metabolite of another opiate that was consumed, for example, high doses of codeine.