Ramen Broth Varieties
I'm not aware of any vegetarian Chinese soup stock. Typical ramen stock is "Superior stock" I think. See this link for a reference recipe: https://www.homemade-chinese-soups.com/soups-stocks.html

My advice would be, if you want to make something that is vegetarian, AND, DELICIOUS then you should throw away your idea of making something Chinese styled and instead stick with the French classic: Equal parts onions, celery and carrots. Simmer those and you will have an absolutely delicious stock. Add some light soy sauce to season, and then see how that fits to your ramen.