I tried to use the calcium carbonate (coral snow) flocking treatment to starve/clog the Vsnails.Not sure what you were doing to kill those snails but you might want to run carbon and dona water change to try and clear whatever out of the water
I have always had a problem with low nutrients in my tanks so I don’t know if the flocking dropped the nutrients levels too much too fast and stressed the corals enough to allow the bacteria to attack. I really don’t know what caused the RTN/STN out break. I lost most of the sps frags in the frag tank and a couple of colonies in the display tank.
I have done water changes and carbon. I am currently discouraged and letting the display tank settle for now. I am hoping the bumblebee snails will prevail over the Vsnails at some point.
I have accepted the fact that the Vsnails are there and have set up a separate Frag system and standalone qt.