Exporting vitamin supplements to Vietnam – legal and paperwork requirements

Question

We are a small business that just has been established few month ago as a wholesaler. We have received orders from Vietnam for vitamin supplements.

Currently, I need to provide Certificate of Free Sale for my customer as required by local authority in Vietnam. However, I still have no experience about legal requirements about exporting vitamin supplements, mainly paperwork.

Could any one please give me some guide such as what I need to prepare before sending products.

thank you so much.