The files you are looking for would most likely be found inside the Chars_M_Body.rez file.
They are usually pretty obviously named. When you find the two, just rename the light vest file so that it has the name of the light vest one, and then inject it back.
For example (these names are made up, but you get the point):
- Vest_M_Light.ltb
- Vest_M_Default.ltb
In this case you would rename ‘Vest_M_Light.ltb to ‘Vest_M_Default.ltb.’ and then inject the newly renamed file into the rez file that you found it in.
Now the game things you are wearing the light vest when you are wearing the have one, so now the speed attributes of the light vest apply to the normal vest, but you still have the protection of the normal vest.