If they wear leather, they are a vegetarian not a vegan.
vegetarian = diet
vegan = lifestyle
You seem to be aware of that, but it is a pet peeve of mine for some reason
That's a very idiosyncratic use of the terms. More conventionally: Vegetarians avoid meat, while vegans avoid animal products all together.
It should be no surprise that people chose to be vegans or vegetarians for a wide variety of different reasons (and thus interpret the restrictions in very different ways). Some consider it to be an ethical or lifestyle issue, but others do it for strictly health reasons.