Acid substitute for tomatoes in black bean & sweet potato stew?

I'm making a black bean and sweet potato stew that calls for 2 small tomatoes, diced. My boyfriend is sensitive to tomatoes, and we also don't have any and I don't want to go to the store. Is there a reasonable substitute? I'm assuming that since it is only 2 small tomatoes, that they are mainly important for the acid. I have a few different vinegars (red, white, rice, cider, balsamic). It is a pressure cooker recipe, in case that is important. Does anybody have any suggestions? Thanks!

Here are the ingredients:

4 cups veg broth
1/2 cup chopped onion
4 cloves garlic
2 carrots
1 large sweet potato
3 stalks celery
[2 small tomatoes]
1/2 tsp cinnamon
1 tsp garam masala
1 cup dried black beans
2 bay leaves
1/2 tsp sea salt (added after pressure cooking)
1/4 tsp black pepper (I will almost certainly add more)