Wandering Chopsticks has a decent crack at making Bo bia. I remember when I blogged this I said the vendor made it with bun (cold noodles). It’s interesting that the real version uses jicama, but that on the streets of Saigon at least one vendor has gotten lazy and gone the bun route. Wandering Chopsticks has more I did not know,
"Bo bia are the Vietnamese spring rolls version of popiah,
a Hokkien-style spring roll popular in Singapore and Malaysia. (If you
sound it out, they both sound the same.) There are some variations with
the filling but in both versions, the spring rolls are stuffed with lap xuong (Chinese sausage), dried shrimp, and jicama. Hokkien cuisine hails from
the Fujian province of China, and I can only assume that immigrants
must have brought the recipe with them when they dispersed to
Singapore, Malaysia, and Vietnam."