Hey! I have N. distortus and vibex and I have found them both to be predatory. Mine will go after almost every new addition that hits the sand, and have even killed and consumed an injured fireshrimp (which had just escaped the grips of a mantis shrimp that I didn't know I had). The moment that poor shrimp went from the rocks to the sand they were all over it, and it took less than 20 minutes to hollow out it's shell! So it seems to me that Nassarius will not only go after any newly introduced scent, but they can also detect injury.
When I added my first maxima clam, they ALL rose from the sand and attacked it. Having seen what they can do to a living animal, I knew that they were feeding on it, so I moved the calm, which was then and is now still perfectly healthy.
What has since worked for me is putting new additions like clams into a supspended net isolation container until the snails calm down. After a couple of days I place the animal in it's permanent position, at which point, as long as it is not injured, dead, or dying snails do not bother it.
I kinda subscribe to the theory that there really is no such thing as "reef safe". It's just a matter of constantly struggling for balance.