Someone asked in a comment how Vietnamese in the thirteenth century viewed the Mongols. I just found this passage in the Đại Việt sử ký toàn thư (15th cent.) for the year 1257:

時元人新取雲南,遊兵略至,無攻取意。當時謂之佛賊。

“At that time the Yuan [meaning the Mongols] had just taken Yunnan. Roaming troops came raiding without the intent to conquer. At that time [people] called them the ‘Buddhist bandits.’”

This is an interesting statement. As far as I know, the only meaning for 佛 is “Buddhist.” The dynasty in Vietnam at that time, the Trần, was by all accounts very interested in Buddhism. So why would the Mongols be given this name?

I would need to do more research on this, but one issue I can think of right away is that prior to conquering Yunnan, the Mongols had been in Tibet where top Mongol commanders had interacted with Tibetan monks. Eventually Tibetan Buddhism would become very important to the Mongols, but that happened later. Nonetheless, perhaps there was some practice which they adopted already at that time, or some objects which they carried with or on themselves? There must have been something either visible, or some practice which the Mongols engaged in which people observed, for people to label them in this way. What was it? Tibetan Buddhism was distinct, so if there was something which the Mongols got or adopted from the Tibetans it probably would have struck the Vietnamese as different.

The image to the left is from the Khâm định Việt sử thông giám cương mục (19th cent.). It is the same passage as that in the Đại Việt sử ký toàn thư with the one difference that in this text the Mongols are referred to as the “Mongols” rather than as the Yuan.