To ask for vegan light lunch ideas?

I would do some warm greek gigantes with bread and salad.

Hummus.

Ratatouille served with couscous, you could add olives, capers and chopped cucumber to the couscous.

Soup....Gazpacho or potato & leek or bortch, with bread, followed by a puy lentil & chickpea salad with avocados vinaigrette.

A selection of baked stuffed vegetables, aubergines, courgettes, tomatoes stuffed with rice & herbs, mushrooms, couscous, mixed beans. Cook during the morning and serve at room temperature with bread to mop up the juice.

Platter of stuffed vine leaves, asparagus vinaigrette, avocado vinaigrette, uncooked tomatoes filled with houmous or aubergine pate, cucumber salad, new potatoes with chopped herbs, rice salad.

Couscous with some lovely roast veg. Just roughly chop a couple of peppers, courgettes, red onions etc with oil. Few garlic cloves. Spice up. Pile the sticky roast veg on the couscous. Serve with a dollop of none dairy yoghurt and some harissa dressing. You can buy harissa paste at most supermarkets. Toast some pittas on the side

Will make everyone happy

If you can get to a Waitrose they have lots of good vegan picky bits that have protein and are quite posh.

I am vegan and your plan of jacket potato (or pitta/flat bread/baguette) with salad and hummus sounds good. If you can add a bean salad or other pre made salad (supermarkets often do a vegan cous cous) or a vegan deli meat (squeaky bean or quorn vegan) all the better.

You're going to get a lot of suggestions for hummmus, falafel and pitta bread!

Including from me. Add a few more picky bits like bean salad, some veg samosas if you feel like heating things up. I'd get a vegan friendly dessert too.

Do they enjoy vegan cheese? If so get some of that for the baked potatoes, the pre grated bags will work. And include a nice soup to your list.

I would go to a decent supermarket and buy a range of picky bits (I know MN hates that phrase!) Vegan bread rolls, hummus, salads, falafels, a tub of soup. A vegan pizza to cut into slices and share. Some breadsticks or similar.

No need for pudding but fruit or some vegan biscuits perhaps.

Soup and a sandwich?

Very easy soup, can be eaten hot or cold. Takes about 20 mins and can be done the day before.

1 courgette per person
1 bunch coriander
oil
boiling water
veg stock cube
(dollop of crème fraiche for omnivores)

Oil in a pan, slice the courgettes and put in the pan to sweat for about 5 mins.

Add the stock cube, coriander (keep a few leaves back) and boiling water.

Cook for 15 mins and blitz.

Serve with crème fraiche and / or a couple of the leaves.

Sandwiches, just get some nice bread (s) and put out salad / veg and maybe some meat or cheese for the non vegan.

If you want to not cook at all then hummus, veg sticks, bread sticks, prepped salad, bread, olives, fruit, guacamole, grapes and maybe some nuts.

You could do a nice fresh salad with veg samosas, onion bhajis, humus, baba ganoush and some flat breads? Check to make sure they are all vegan but a lot will be. Or veg soup, bread and get some plant based spread instead of butter for the vegan?

Hi OP, it's lovely that you're thinking of your BIL. I would stick with your original plan, and just add a couple of lovely pre-made bean/lentil salads and some hummus into the mix. As a vegan myself, I would be happy with that!
Just so you know, there's a vegan forum on Mumsnet so you might get good suggestions there too.

Falafel?

with houmus, salad stuff and pita/ flatbread. Omnivores could have the same or cold meat in place of the falafel.

What about a shop-bought vegan lentil curry - MS/Waitrose probably do them.

Served with flatbreads, chutney, raita etc.