[Solved] Hanbok LODS 2-3 glitch

Hi. This happens because the vertex groups number per meshgroup is exceeded in all lods. That said, why start it from hanbok exactly?
Your garment has no shoes, it does not need to be made this way. When choosing a reference, your priority should always be the lengths, not puffiness.

- clone a dress with a skirt of similar length, preferably puffy as well. It does not need to be base game. For example, yfBody_GP08DressOffShoulder. Clone, export the blend, open;
- join all meshgroups and delete all legs from it, rename to Reference;
- append your LOD 0 dress, important: delete all parts of the body and all vertex groups, rename to Garment;

- using Data transfer, transfer weights, uv_1 and vertex paint from the reference to the garment, then Clean with Subset set to All groups and Limit total;

- add the body, delete the reference, save;
- clone a lingerie set, import your blend there;
- export lod1 from this package, open, append your lod0;
- delete the body from the appended mesh, select the lingerie set, delete all areas covered by clothing;
- decimate the dress, apply, join with the body, save, import back;
- try decimating the whole lod1 to make lod2 and lod3, then check how they behave in-game. If they distort, make lod2 the same way you did with lod1.