Fix a bad vegetable soup
With such a random collection of ingredients, I'm hesitant to suggest anything lest it conflict with one of the flavors.  With that in mind, add extra ingredients a little at a time to make sure it doesn't go overboard or taste jarring with something already in there.

Suggestions to improve the flavor:


Celery salt or celery seed (preferably ground).  Fresh celery is normally part of the mirepoix in veg soup, but it's probably not a good idea to add to a cooked soup, so we're adding the next best thing.  It is amazing the difference celery can make.
White wine vinegar (for acidity and a richer flavor, and pairing with the cabbage and onions)
Dry vermouth or white wine. Some flavor compounds are more soluble in alcohol than water, so this can help bring to the front additional flavors that are already present but undetectable.
A jigger of sherry or cooking sherry (the latter has salt, so add it before salting the soup). This is a common suggestion when serving canned soups to guests, to make the soup taste fancier.
Rosemary, thyme, and parsley flakes/stems (adds aromatic and herbal flavors)
Worcestershire sauce (use carefully, it's potent stuff) -- this adds umami and spiciness
Paprika and a pinch or two of cayenne -- adds warmth and back heat, and smokiness
Dijon mustard (use sparingly).  Adds front heat and a little richness to the soup
A little more salt and pepper
Parmesan or parmesan rind (for the rind, plan on simmering for a long time to extract flavor).  Self-explanatory here.
Balsamic vinegar -- adds sweetness, acid, body, and umami.  Use caution, it may conflict with the cabbage here.