Vegans don't eat fish. There is no nuance that allows one to be a vegan and eat fish. They are mutually exclusive, like teetotal whiskey drinker, or intelligent UKIP voter.
Exactly. Isn't a fish-eating vegan technically an oxymoron?
It's so much more stark than the previous example of 'contraceptive-using Catholic', since there are a wide number of behaviours, beliefs and observances which define a Catholic. But the English definition of 'vegan' is 'a person who does not eat or use animal products' (OED, Merriam-Webster, Cambridge, various vegan groups). It begins and ends with consumption.
There might be more wiggle room if the accepted definition predominantly referred to beliefs, ethics or intentions. But it doesn't.
(Although, somehow, 'fish-eating vegetarian' doesn't wind me up much. It just seems a bit clearer okay, no meat, but fish okay and is more of a convenient, shorthand summary than anything else. Whereas a vegan who eats fish... with a much longer list of restricted items, how are you supposed to anticipate what's acceptable, if fish is back on the plate?)