The normal XLB dumplings are made with a cold filling that is rich in gelatin, and hence paste like when assembled. When cooked that gelatin liquefies, producing the soup interior. How could the same effect be achieved without meat gelatin?
agar agar is a commonly used gelling agent in Chinese cooking. But I'm not sure if it has the right melting properties for this use. It is usually used to make something that gells at tropical room temperature.
You might want to try the Chinese vegetarian restaurants. 3G Vegetarian on Cambie has vege sui mai for example. Like paulj said, XLB is made with a pork-based gelatin. I don't know if agar will work-it surely will not taste the same though.