On the subject of vegetarian cheese, be aware that parmesan is not vegetarian. However, you can get a parmesan style cheese that is vegetarian. We've had this from Tesco before but other stores may sell it.
If you are looking for something a little more substantial rather than just 'nibbly bits' I have just picked up the Waitrose's free Christmas magazine. It has a section, albeit a little short, for vegetarian Christmas. One of the recipes, which I think would probably be OK cold, is called Lebanese Pizza and has aubergine, houmous, onions, herbs, pine nuts, pomegranate seeds and feta cheese in it.
Something we've done in the past is a tart made with a puff pastry rectangle base with a 'border' scored around it. The middle is then filled by layering slices of tomato, roast pepper and/or other veg and mozzarella (although is this veggie, even the non-Buffallo type?). Bake in the oven until pastry cooked and then serve with basil leaves scattered on top. You could always bake the pastry 'case' first then finish with the layers to serve cold. It may be better to make this up as near to the time as possible so that the liquid from the mozzarella doesn't make the pastry go too soggy. Also, we have had the cooked tart cold as leftovers and it tasted perfectly fine.
You can also do something similar using caramelised onions and thyme.