While my TL that I am working on has a PoD later than your set year, it is still applicable to make one of them becomes a powerful empire.
Anyway, it's conceivable for Dai Viet to become much more powerful by playing the right card. Say the Nguyen Lords became more open to the West and they traded more weapons, ideas, which allowed them to expand faster than OTL and reached OTL Southern Vietnam by 1700. OTOH, the Trinh - Nguyen civil war was shortened due to the Nguyen Lords, before 1600, did not provoke the Trinh Lords, delayed the civil war for some time, which brought in more manpower for the Nguyen. The rapid expansion pitted the Nguyen with Ayutthaya, which they managed to defeat by using more Western weapons and had much better defense (in OTL the Nguyen did defeat Ayutthaya, don't underestimate the power of Dai Viet).
The Nguyen Lords also embarked more into trading and manufacturing compared to the Trinh Lords, which allowed them to outpace the Trinh economically and production-wise. With a more capitalist economy that was hungry for raw materials, the Nguyen attacked the Trinh and won in 1750s, reunited Dai Viet. Over the next few decades, they actively pursued an open policy towards the West and gradually westernized in a unique manner. By 1800s they became the first country outside of Europe and America to adopt the European model for science and capitalism, enabling them to industrialize rapidly. European empire-building became the #1 foreign policy of Dai Viet and they decided to invade neighboring countries to prevent other powers from claiming these lands. In 1840, this goal was achieved and Dai Viet dominated the islands of Borneo, Sumatra, and mainland regions like Cambodia, Laos and parts of Siam.
I must note that a PoD that far could have a lot of ramifications that with my limited time, I haven't fully conceived a detail picture for this scenario. Dai Viet could go down many paths, especially for a wild card like the Nguyen Lords, who in OTL played with the West in a manner that later angered them, rather than fully exploited their powers for Dai Viet's purpose. As well, I made a whole lot of assumptions in the previous paragraphs just to form a bare back-bone for the challenge posted.
