I have had this issue happen to me on several occasions, where I place a viewport on a sheet (viewport of just a simple line drawing on the design layer, no symbols or anything fancy, although sometimes it also happens when it's a viewport of a referenced file) and after working on the file for a while all of a sudden the content of the viewport is not visible anymore. It only shows up when I zoom in or out, but disappears when I stop.
It happens completely at random and for several files. I have checked all obvious things, like if all layers and classes are on, etc. I don't think it is a setting that is wrong or if I accidentally gave a wrong command, but more that it is either a bug on my computer or my vectorworks program? I have had someone tell me it might be a problem with the graphic card?