There is only one way I can introduce you.
Japanese government (The Ministry of Health, Labor and Welfare) makes "Working condition consultation hot line" for people who speak foreign languages, so your problem can be shared in Vietnamese language at the hotline. The free telephone numbers are showed in the URL below.
I'll be glad if the information is helpful for you.
Here's a translation for everyone!
English - My company required me to work overtime and they have reduced my salary without my permission.
This is our understanding of the question. Hope this helps!
This information also could solve your problem.
Telephone Consultation Service for Foreign Workers
I hope this would help you.
If we understood your situation correctly, would it be possible for you to consult the services for Foreign Workers with the Ministry of Health, Labor and Welfare? If you have any trouble related to working conditions, the following webpage might help you. It has information on counseling services and hotlines for foreign workers.
Should you have any further questions please contact us again. We hope everything works out for you.