Outfit explodes in game, going insane

Hi. You needed to delete all vertex groups before transferring weights.

Also, unrelated to the problem, but the is no proper uv_1 and vertex paint, so the garment wont morph. In uv_0 the uvs are in the space meant for shoes, so there will be conflicts with shoes. You removed too much of the body, so it has to be replaced.
- clone ymBody_EF01SwimPanelRuching, export the blend, open, rename the garment to Reference;
- append the yukata from fixedyukata.blend, delete all vertex groups from it ( v button under + _ );
- duplicate the reference, rename to Body, remove doubles, delete all areas covered by the garment;
- re-arrange uvs of the yukata, rebake textures, delete the original map, rename uv map to uv_0;
- transfer vertex groups and uv_1 to the garment from the reference;
- in Edit mode select everything with A, Clean with All groups and Limit total;
- vertex paint everything with 00FF00;
- join the garment with the body, give it 0000 cut, delete the reference
- save, import back into ymBody_EF01SwimPanelRuching

I'll explain everything gradually then
uvs are two dimensional ( length, width ) representation of a three dimensional ( length, width, high ) object. They are used to define how textures are projected on the surface
When you move uvs from one area into another, what they project changes, so your goal is to move both uvs ( so that they are in a new location ) and the garment's texture ( so that the garment still displays the right information )
Previously, you have already done it manually. You moved both uvs and textures. Unfortunately, the space is very limited, so you would have to scale down the whole uv layout even more to fit it in that area without conflicting with shoes. This would lead to a dramatic loss of quality ( one pixel can display only one color => less pixels to display an image = worse image quality )
So, to minimize this loss, you'd want to move these uv islands individually instead of the whole thing. Then it would be easier to utilize all available uv space

Anyway, if you delete more areas of the body covered by clothing, you will be able to move all uv layout and textures manually to the the torso area, there is enough space there


But to clear up how the automatic re-arrangement should be done in your case:
1. Create a new uv map by duplicating an existing one. There should be an original uv map that is displaying an original texture and a second uv map that contains re-arranged uvs;

2. Select the body, Shift-select the garment,  select the second uv map, select all uvs with A, assign the template. In the side panel enable Draw other objects with Filter set to All to see the body's uvs;
3. Press S Y 0.5 to brings uvs to the original size;
4. Put them into the space meant for tops and bottoms, making sure to avoid body parts;
5. Create a new uv map with transparent alpha;
6. Re-bake textures;
7. Save the result;

8. Delete the original uv map, rename the remaining one to uv_0