The EU have negotiated Free Trade Agreements with many countries, some of these take years of negotiations. A new one finalised this year was the EU-Vietnam FTA. This Trade Agreement will mean EU exports to Vietnam and vice versa will benefit from preferential tariff treatment, once it qualifies based on the submission of statement of Origin.
Exporters should be registered in the EU's Rex system or EUR1 document issued by competent authorities
any consignments under EUR6000.00 should have an origin declaration made out by an approxed exporter within Article 20 of Protocol 1.
For further information you can find it on the below link.
While this is the newest FTA there are plenty others that may be applicable to you
In addition to FTA's some countries qualify for GSP (Generalized System Preference).