I see vegetable broth for sale in cans/boxes at grocery store. Growing up I remember one of the churches did creamed cod and potatoes for a meal once or twice a year. Wasn't Catholic, but I wish I could remember.
Yes, but "Grandma" probably didn't use it.
When I was growing up we didn't have fish gravy (tuna casserole doesn't count), but we ate a lot of <whatever> with milk gravy over diced potatoes. Mama used bouillon cubes; beef or chicken. I'm pretty sure she learned that from her mom, or from Home Economics class.
I'm just saying if you're trying to replicate something from 50+ years ago, use bouillon cubes. Or add carrots, celery, onion, and bayleaf to the water when you cook the dried fish and use that broth. Vegetable broth might be better but it's unlikely to be authentic. (if you have shrimp shells, they make excellent fish broth and that might be authentic too.)