@shantrischool
Welcome to expat.com, shantrischool, and thank you for your post!
The issue of volunteering in VN has been a topic of discussion on this forum many times.
I believe the process in acquiring a DN1 Volunteer Visa is even a bit more complicated than the Working Visa, in that the Volunteer Visa requires not only the approval of VN immigration, but also of the local community authority in the area/region where the volunteering will be offered.
I could be wrong on this, and if so I am open to being corrected.
Thank you again for your post, and once more, welcome to expat.com