I havent read his book yet, but from what I have gathered, the prince he met in real life was Võ Vương, also known as Nguyễn Phúc Khoát, so I'm guessing that the prince he was describing was Nguyễn Phúc Khoát.
For a bit of context, Vietnam was in the midst of a 300-year civil war in the 17th century. The country was divided into 2 domains, the Trịnh lords in the north and the Nguyễn lords in the central and south. Both clans were theoretically under the command of the Lê emperor, but in reality, the Lê emperor was in name only and the 2 clans were the actual warlords in charge of the 2 domains. This situation mirrors the shogunate system of Japan in the same period.
According to the version of Confucianism of the Vietnamese royal court at the time, there were many methods to rule that the emperors had to follow at the time: military, bureaucracy, morality, religion, etc. Generally, those methods were bundled into a specific set of law called Hồng Đức code, written by Lê Thánh Tông during the 15th century. The Hồng Đức code represented the rule of law + legalism policy of the Lê dynasty.
When the 2 clans took over, the 2 clans were both military warlords. However, as their rules in the 2 domains extended, the Lê emperors started to only serve ceremonial purposes, the 2 clans slowly took over the duties of bureaucracy and philosopher rulers as well of their respective domains, taking on the set of duties of full-fledged emperors for their domains per the Hồng Đức law code, if not in name.
Some lords were better than others at actually doing those duties, but Nguyễn Phúc Khoát was one of the good ones among the Nguyễn lords. Before Nguyễn Phúc Khoát, the Nguyễn lords was slowly losing control of their domains to local warlords, especially in the frontier regions in the deep south and mountainous regions. Nguyễn Phúc Khoát was the one to attempt to resurrect the control of the Nguyễn line in the central and south. He followed the Confucianist doctrines in the Hồng Đức law code closely and emphasized heavily on the rule of law along with morality rule.
In the end, his success was limited and dismantled by his incompetent successors, so his domain fell to local warlords anyway.
In any case, while I dont have access to Poivre's writing, I highly suspect that Poivre mistook the Confucian ruling doctrines in the Hồng Đức law code that Nguyễn Phúc Khoát followed as philosophy and ended up calling him "philosopher king".
It's best to hear it from Poivre himself, because what Poivre saw was not neccessary what Vietnamese historians (who grew up in a Confucian environment) saw:
The six first kings, founders of this monarchy, governed the nation as a father governs his family ; they established the laws of nature alone; they themselves paid the first obedience to them. Chiefs of an immense family of laborers, they gave the first example of labor; they honored and encouraged agriculture, as the most useful and honorable employment of mankind. They required from their subjects only a small annual free-gift, to defray the expense of their defensive war against their Tonquinese enemies.
This imposition was regulated , by way of poll-tax, with the greatest equity. Every man, able to labor the ground, paid into the magistrate, on account of the prince, a small sum proportioned to the strength of his constitution, and the vigor of his arm, and nothing more. It was under their reign, that this nation multiplied so surpriseingly, in consequence of the plenty furnished by the culture of their fields. Whilst they reigned, the treaties entered into on the banks of the river, which separates Tonquin from Cochinchina, between the chiefs of their family and those who followed them in their retreat, were most religiously observed . It is to this reciprocal fidelity that Cochinchina owes its present flourishing state with regard to power, population and agriculture.