Can an employer make you work then tell you he is paying you as a vacation day?

I was in an auto accident where i crashed the company vehicle. I was traveling in a congested area at 5 car lengths following length everyone panic stopped and my foot once on the brake slipped. I got back on it and pressed hard but since i was in a large pickup/work truck. I hit the person in from of me. We are a union shop however I am afraid to say anything without knowing my rights. This new boss is ex military and does what he wants. He wrote me up as a warning, made me clean the outside of the building, do drivers ed videos, transfer tools to a spare truck. Then he told me he was not going to pay me for it and today I would be paid as a vacation day? I had no ticket from the state trooper nothing saying i was doing anything reckless. I have never sued anyone and really don't plan on it I just want this guy to follow the law. I would appreciate any and all advice on how I should proceed. Thank you so much in advance.