Tell them to bring their own expensive vegan stuff then, sausages, milk, cheese, as you don't want to be left with stuff you won't eat, thats fair enough.
Then cook stuff that works both ways, sausage and mash, just split it and add their vegan butter/cheese/milk to theirs and your butter/cheese/milk etc to yours.
All sorts of things that start with minced beef you could do the minced beef seperately and then split the veggie/tomatoey/oniony base and add the browned mince to your portion. As all those things tend to simmer for a while yours will still taste just fine and you can throw a handful of dry lentils into theirs.
If they object - tell them not to bother coming, in hindsight you're too busy/tired/just can't be arsed.
If they fail to bring with them what they need then they'll just have to have whatever the base meal is, without the meat/meat substitute.
They do sound like absolute dicks though so I think I'd tell them I'm away that weekend.. for the next decade at least. It's nothing to do with them being vegan, it's entirely to do with them being twats.