Certainly they were the first to use cobalt and NOT in a lead based glaze but other nations soon adapted to using it as well.
The #Vietnamese potters tended to use a darker more indigo underglaze and smaller shapes. Its a lesser known ware possibly because the export trade peaked and then dropped off during the 16th and 17th centuries as the Vietnamese like the Japanese exploited the disruptions to trade caused by conflicts between a failing Ming dynasty and the spread southwards of the Manchus.
Plus the introduction of polychrome wares and the European ceramics industry learning to adapt white EW clays to create their own wares reduced demand.
Here's some pictures to enjoy. The underglaze pigments seem a little coarser but the brushwork is often delightly fluid.
The pieces I have shown here actually date back to the 15th century.
This final image although looking more "chinese" is indeed Vietnamese!
Some sites use the word Annamese rather than Vietnamese.