if your doing vrm you need to use MTOON shaders. if your using GTLF (why tf does this exist?) or standard its gonna be all ugly with over shadow. vrm models use a shader called MTOON. if you have a vrm environment set up in your unity (i assume you do if your messing with that format) you also need to export an import the models again to normalize it so that you can set up your physics (if you want any). it adds the secondary section where you place all the spring bones for the vrm model. also bare in mind not all models can cleanly convert. some are just not able to be used as a vrm due to how there meshes and matrials are set up as your working with a shader that proly don't support what the model needs. its a 50/50 gamble when you try to convert stuff to vrm.
most of the time its fine but theres also times where it just dont work. so bear this is mind. in your case it sounds like your not useing mtoon and that would explain why its being shadowed like that