Dolby Prologic (Dolby Surround) and now PrologicII are encoded on stereo signals and decoded by the receiver. These are all matrix surround modes and while not as good as discrete, PLII does a decent job of simulating surround. When they say simulated surround on the box, that likely means some proprietary type of matrix surround that should be able to be decoded by most receivers; meaning it is not true PL, but can be processed by a PL decoder or something like DTS:NEO6 or some other stereo decoder. "Virtual surround" often refers to something trying to recreate simulated surround from just the two front channels, but it is hard to say since they could mean anything by it.