My daughter backpacked through Viet Nam (and SE Asia) in 2002. She ate most of her food at street vendors in open markets. Did not get sick once.
She said she stood back and watched a little while. Chose the vendors with the longest lines and least flies.
I respect your daughter for being so adventurous Anthony Bourdain style in Vietnam. Though unlike her, Anthony has many staff to steer him clear of the pestilence traps.