Maybe someone can help me out here, but I didn't think the soft vests (such as the OP was referring to) could protect against stabbing knife attacks. I know mine can't (Level IIIA).
I just looked into it, and I guess I just answered my own question.
Body Armor Levels
"Soft body armor is not designed to protect against rifle rounds or edged or pointed weapons." (In the green box, about 2/3 of the way down.)