Why not "Who did this work?" I thought lam viec meant work. It also makes sense as in when you see a painting or sculpture and you ask "Who did this work?"
Consider it in chunks like this: Who / did / this (in reference to an affair or matter) Ai / đã làm / việc này. Việc can mean work, sure, but it can can mean a matter, affair, something that happened, etc. It has more meanings than work or job. Example: việc gì đã xảy ra thế? What has happened? Việc gì = what (roughly, in reference to the occurrence) đã xảy ra = has occurred/happened
Just stuff to chew on for later
It says ''viec nay'' is a different word here. Correct me if I'm wrong but I guess, ''viec'' has no relation with ''lam'' in this sentence.