Well fancy meeting you here!
As I said in the other thread, this is usually a sign that you either moved something (the vst .dll file or maybe its associated config/sample/instrument files) or you haven't added the path where you actually installed the plug and then cleared and re-scanned your plugin paths within reaper.
Just to clarify, are you saying that in each case you used the manufacturer's supplied .exe file to manage the installation of the plugin?
I still put my money on an incorrect "read" from Reaper's Preferences/Options/Plugins/vst menu.