Do they even like oat milk? I can't stand it, makes my tea taste like porridge!
Far too expensive as well. I buy cheap "Soya milk no sugars" from Aldi which is bland tasting and cooks the best in sauces. Buy a block of Stork which you can use in baking when they've gone.
I don't eat meat fish or eggs, but eat butter and cheese. My daughter is a strict vegan. No wool in clothes, no honey in food etc. So have to adapt when she visits.
You could cook what you might consider a side dish and a vegan would find it acceptable. Cauliflower cheese for example. The cheese sauce being made with Stork, soya milk and Engevita nutritional yeast flakes, which taste like cheese.
I cook a vegan version of aubergine parmigiana, stuffed peppers, lasagne etc.
A Delia Smith recipe for fried halloumi and dressing, substituting cheese with Tofu, which makes a great starter. Curry, leaving out meat for theirs and adding chickpeas. None of those are expensive and all cook ahead.
I think your friends sound tight as well as selfish.
Selfish as vegan cooking has to be learned. I'd call myself a keen cook but plant based spreads and milk act differently in cooking for instance.
Tight, unless they bring you goodies and take you out for a meal or even a case of wine.
As they're close enough friends to stay regularly, I'd ask them straight, "Please bring your own Vegan food as I find it too much to cope with along with cooking our meals."