Being vegan and being on a plant-based diet have a lot in common. You may be surprised - some people act as if the two are opposed, even possibly enemies. That doesn't have to be the case. They are, however, different, and these differences need to be made clear.
Are mostly motivated by moral concerns relating to animals, although there may be other motivations in there as well.
Prioritise avoidance of animal products
Also avoid animal tested products
Generally build up a lifestyle that excludes these things as far as reasonably possible.
May use other arguments - e.g. health, the environment - to promote veganism but these tend to be what we in the business call adjacent concepts. They help to shape the way veganism progresses, but you could remove them from the equation without the whole thing collapsing.
A plant-based diet:
Is just a diet. People may go beyond this but it isn't intrinsic.
Is frequently motivated by health concerns. Can have a moral element but doesn't have to.
Usually excludes animal products, but this isn't always the main priority. (For example, as the motivation is health-based someone might choose wholemeal bread with honey over white bread without, whereas a vegan acting on moral grounds would suck it up and eat the white or something else.)
Excludes processed food, sugar, certain vegetable oils and refined grains as far as possible - foods that are vegan but not healthy.
The confusion comes when 'vegan' becomes a catch-all term. The strife comes when people call themselves vegan and proudly proclaim their love of honey. More strife comes when vegans don't accept that someone who isn't vegan finding a different name for what they are doing is a good thing. Seriously, we shouldn't complain when someone who isn't vegan and has no wish to be stops calling themselves that.
Personally, I would love to be able to be plant-based as well as vegan. I don't, however, have the willpower. The moral bit I can do, in fact I can cut down on things for health reasons and still be happy, but absolutely cutting out sources of pleasure because I might live a little bit longer isn't going to happen.