you can use any stock really, veg cube, veg stock powder, chicken cube or chicken stock powder. I tend to use both of these, depending what first falls out of the cupboard, beef stock sometimes gives the soup a more meatier taste. I would refrain from using up stock from any roasted meats, may be a little too much fat in that.
To be honest were soup is concerned, there isn't really any set recipe, in our house, it depends on whether I've been prepared and bought stuff in, or whether the veg sat in the bowl is looking tired and I don't want to waste it, and freeze the soup for a quick tea!! My freezer is full of foil tins of soup and cottage pies etc!
Add some lentils or rice for a thicker soup, salt and pepper and herbs to taste. Can't go wrong with oregano!!