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You're very much presenting a doomed if you do, doomed if you don't scenario. As a picky eater (non vegan) I've found that if you're unwilling to eat something a host made they get upset. They may never directly say it, but there's always some negativity. Bringing your own food is considered disrespectful, for better or worse. The "rules" of politeness and manners in these types of situations sadly don't follow much logic.

So, if a host knows there is a vegan and prepares a dish with meat in it, what can the vegan do? Separating it out doesn't seem good, there's still probably meat juices and things they don't want mixed in. Bringing their own food will seem rude and disrespectful to many hosts. Refusing to eat will likely be viewed as them being overdramatic by the hosts.

So while it's true that the host has no true obligation to prepare something separate, they're still knowingly putting the vegan guest into a catch 22 scenario. The host isn't under obligation to provide food at all but they are and they're knowingly choosing something that a guest can't pick due to dietary restrictions.

I'm basing this on the types of gatherings and families I grew up around in the American southeast. It's not universal, of course. Even here it isn't. But I also believe the types of hosts who would be tolerant of you not eating or bringing your own food are also the ones who would be polite enough to separate the meat before and cook them separately or they'd make something the vegan guest can eat just in general.