It is a good question because you have not mentioned words like ‘food’ and ‘diet’ in it. Veganism is much more than just strict vegetarianism. It involves eating fully plant-based food items, and not using or not taking advantage of products derived from animals. As far as I know, a vegan does not even use medication that is tested on animals, for ethical reasons.
On the other hand, the term ‘plant-based’ only refers to a diet, which mainly or solely involves plant foods. One who follows a plant-based diet has limited to zero products derived from animals. It may not affect lifestyle aspects such as clothing and personal care items quite as veganism does. Another difference is that veganism is becoming a growing international movement. A vegan even considers the potential impact of animal derivatives on the environment and their main objective is to end animal cruelty.