I'm running interchangeably between two systems:   Mac Pro (Mid 2010) w/ 16 GB RAM running macOS Sierra & MacBook Pro (Retina, 15-inch, Mid 2014) w/ 16 GB RAM running macOS High Sierra   I have this same issue running Vectorworks 2018 SP2 on both machines.   When rendering (doing anything really, but rendering is the main culprit) I eventually reach a point where the application pauses and I get a message stating that "Your system has run out of application