I use soy sauce, dried shiitake mushrooms, mirin and kelp or I use seaweed dashi, soy sauce and any umami spice you can find. The viet shops have some vegetable based broths made with shitake that emulate meat flavour. Or you can use any meat broth or fake meat broth in your country.

During the hot months I make a cold noodle soup with thin starch noodles (like mung bean or rice), ginger, seaweed, tofu, crispy chilli and lime juice/vinegar. I top it with julienned cucumber, carrots and scallion :D