According to Wikinonsense, in Guangxi, sawtooth coriander is known as 假香荽 (jiǎ xiāng suī) or 'fake coriander'. Unfortunately, they seem to have forgotten to tell anyone in Guangxi. No one I've spoken to, including local chefs, has heard of it.
Now I just need to find rau răm.
It is so frustrating living right next to Vietnam but being so unable to source so many of its iconic ingredients.