Romance of the Three Kingdoms and Journey to the West. Until this day these two works are still more popular than any Vietnamese literary works. Some Vietnamese works that are popular but not as popular as those two are 嶺南摭怪 Lĩnh Nam chích quái, 傳奇漫錄 Truyền kỳ mạn lục, 安南一統志 An Nam nhất thống chí (also known as 皇黎一統志 Hoàng Lê nhất thống chí), 公餘捷記 Công dư tiệp ký. All the mentioned work were written in Classical Chinese, for Vietnamese-language works there are two most famous works 蓼雲仙 Lục Vân Tiên and 斷腸新聲 Đoạn trường tân thanh (more popularly known as 傳翹 Truyện Kiều)

You could also check out 歷朝憲章類誌 Lịch triều hiến chương loại chí (Categorized Records of the Institutions of Successive Dynasties), it's an encyclopedia written before French imperial rule. It talks about many subjects including notable works of literature (from Confucian point of view)