"Virtual Emulator" is a term I picked up somewhere, I think it may have been in some of the GameLauncher documentation at one point.
By creating new sets of .cfg, .map, and .rom files, you can add other emulators to GameLauncher that it is not already configured for. An example is O2Em, which is an Odyssey2 console emulator. I added the .cfg, .map files to run this DOS based emulator.
By using .rom files you can create multiple configurations for the same emulator. In your case you would have one set for horizontal games and another for vertical games. These two set ups would be the "Virtual Emulators" because they are both MAME but they appear in game launcher as to seperate emulators.
Again, I am not sure where I came up with the term virtual emulator, it may have been in the older documentation, or on David Dribins web site, or it may have been in one of the emails that I got from David, when I was trying to figure out how to launch .bat files from GL.