I always work on the assumption that there needs to be enough of the veggie stuff for everyone, plus meat and fish for those that will partake.
I think you can provide some veggie options at little extra cost/hassle
Spanish omelette is easy and cheap to do. At a push you can buy it ready made, Sainsburys do one that isn't bad. Or fancy it up into a frittata - loads of recipes out there.
Lots of olives - you can get the Spanish ones with manchego to go with the tapas theme. I'd also get some of the stuffed peppers a pp mentioned, or a jar of roasted peppers that you can then chargrill in a griddle pan.
I'd do a tomato salsa style salad (Google delias recipe for this, it's awesome) plus a couple of other salad options. Maybe just a bag of rocket salad, some shaved Parmesan, oil and balsamic.
Dips - easy to make or shop bought for ease.
A few crisp/ chip/ bread options. Veggie sticks for dipping.
Patatas bravas or new potatoes roasted in oil with your choice of flavourings (I love garlic and rosemary).
A tiny bit more fiddly but I love pea and mint bruschetta and always make it for parties. On phone so can't post link but it comes up if you google.
Again, for ease, you could buy a couple of bags of ready prepped roasting veg, roast that with some garlic/herbs, add cherry tomatoes for the last 10 mins or so, stir through cooked pasta and crumble in some feta - one of my favourite dishes and much appreciated by people I've made it for including confirmed meat lovers.
Or roast butternut squash and sweet potato, transfer to pan with veggie stock, blitz with hand blender, stir in cream if you fancy, makes an amazing and filling soup.