Is there a way to hide the Help/Diagnostics tab on the menubar within the Vision client or restrict it to specific users?
Not in the menubar yourself, but you can put a button or some other control in the app to open it with scripting.
https://docs.inductiveautomation.com/display/DOC81/system.gui.openDiagnostics
So the only way to disable it is to hide the whole menubar?
If you dig into Java Swing, you should be able to delete that item from the menu using a startup script.