First, I am not vegan. Second, this is not true. If it were true that they could easily obtain bison meat, why did they also eat a variety of other smaller, easier to kill animals? The bison hunt was a huge undertaking. It may be romantic to think of tough indians riding around killing buffalo on a daily basis but this was not the case. This is also precisely why one of the main ways bison meat was consumed was through drying it and mixing it because it was a way to preserve it through the long winters so that they would not starve, because contrary to the action movie you are thinking up in your head that life was like back then, meals were often times few and far between.
Also, plant foods can be quite filling, especially when its simply a matter of going out picking as much as you want. Yes, there were seasons, plant foods were not available year round. Just like bison were not just standing around waiting to be killed.