I haven't heard the Marshall you're referring to, but the old light blue Boss CE-2 chorus pedal is like that. Then Voodoo Labs duplicated it for about $150+ or so. Most sellers know what they have, so instead of being a $30 used pedal it's an $80-$200 specialty item. But sometimes it's a pawn shop surprise, and e-bay prices are all over the place.
If that Marshall is stereo, then you can't duplicate it with a mono device. Plus, you have to determine if the VS has "stereo" chorus, where the sound modulates on both sides, or if it's like most of the Peavey/Crate/Fender princeton chorus amps where one speaker stays "dry" and the other modulates. They're very different from eachother.
Now I wish I HAVE heard that Marshall because I could tell you what it's doing.