Not Vegan, but often have to have the Vegan option due to dairy/lactose allergy/intolerance...
I look for lots of variety and freshness. A Vegan beige/brown/taupe combo is no less disappointing than a processed meat and pastry/breadcrumbs based one. and Vegan Quorn tastes like crap whatever you do with it
Vegetables, veggies, veg. Separate dishes, so there's a lot of choice (and no chance of those who have already had their standard food grabbing all of the lovely veg based stuff as well).
Tabbouleh is brilliant. As long as you use a huge bunch of parsley and not the homeopathic doses most people do. Using Bulghur Wheat (like you're supposed to) is far nicer than couscous in pretty much every situation in the world. And I think polenta is disgusting, no matter what else is added to it to try and hide it's taste and texture.
Provide lemons, limes, other dressings to add as liked. Something pretending to be a creamy dressing could be revolting looking at you, cashew or almond anything and if it's all over the animal/dairy free stuff, you're stuck with it.
Curries are great and don't need a cream replacement - potatoes or potato starch thicken things up brilliantly. Making little fritters of onion, carrot and spices with some chickpea flour (water to make a light batter) and then baking in the oven are far nicer than a single, massive and greasy onion bhaji. Mini naans (check there isn't yoghurt added or make your own flatbreads) are brilliant, especially if toasted on an open fire.
Fruit needs to be available separately, not just chucked into a bowl and smooshed up together. As this can be equally appealing to the dairy lovers, get far, far more than that expected for two people. Strawberries are always appreciated.
Be aware of bread/rolls. Whilst the cheapie 'brioche' may not have it in there and the sugar content is horrid anyhow, the 'better' ones will have dairy added (or even actual butter).
And if there's a barbecue, a) don't cook meat on the rack and then expect vegans to want anything from there and b) make space for half cobs of corn on skewers that can then demolished with a spicy dressing. Opportunities for chilli and chilli flakes are your friend.
Obviously, not all of these go together at once, but the general principles are that the 'sides' are hugely important, as you're not just chucking them in as a mere distraction from a main event of animal products, vegans don't necessarily demand ersatz meat products when there are vegetables, grains, pulses and thousands of years of animal free cuisines in human history to choose from - and the non vegans are likely to allow be very, very interested in what's to offer, so over-cater rather than err on the side(dishes) of caution.