FWIW, "Render as" is not an option in the 'destructive' editing mode. It is an option though when working on a non-destructive Sound Forge 'project' (which will have a <.frg> project file and folder).
That said, this MP3 issue seems to be a common query with SF and VP, search this group and the Vegas Pro audio forum.
Otherwise, you can try resetting Sound Forge: (File> Reset All preferences and cached data).. or uninstall > reinstall, using a third-party uninstaller or cleaning left-over folders, data and reg entries prior to reinstall.