doctor doesn't believe in B12 supplements for vegans??

I just went to my doctor primarily because of memory issues that have been getting worse over the last ~6 months. I told him that I'm vegan and that I've never taken B12 supplements which caused me to believe that my memory problems might be coming from that direction. He then told me that vegans don't need any B12 supplements and that a balanced vegan diet would be sufficient for B12 intake. I'm baffled because everything I've found online including experiences from other vegans and dietitians made me think that B12 supplements are vital in a vegan diet so I don't really know that to think right now and maybe someone can give me some insight, would be very much appreciated!