A lot of industries have local trade associations where manufacturers, retailers, and distributors can come together. This may be the first step to find a reliable business partner in the particular country. The most popular Vietnamese trade associations include The Vietnam Chamber of Commerce and Industry and The Union of Associations of Industry and Commerce. The first one is an organization which assembles and represents business community, entrepreneurs, employers and business associations in Vietnam. The purpose of the Chamber is to develop and support the business community, promote economic and technological cooperation with foreign partners. The Union of Associations of Industry and Commerce is an organization of businesses and companies in which aim is to develop businesses in Vietnam. You can also use trade associations to learn how other producers sell their products, what distributor do they use, and after that adopt their best practice.
You can use Facebook or LinkedIn to find an appropriate distributor for yourself. Joining industry groups, forums, and other professional networks can be very beneficial too. More experienced business owners in your area are often the best source of information. Spend some time to build networks that may help you find the best distributors for your products. Online forums can be a good source of free information as well: you can get help from other people with experience in your market or industry and take into consideration their experience.