When people talk about virtual displays, they are referring to monitors, ie: wanting to replace a three monitor desktop setup with virtual displays. This is not possible.
You are conflating monitors with app windows. There's a big difference and many unanswered questions. Will Vision Pro apps support multiple windows like the Mac, for example? Or will Vision Pro apps be more like iPad apps that are restricted to one window? Or do Vision Pro apps need "windows" at all?
Yes, a single Mac display can be brought into Vision Pro (basically AirPlay) and of course that Mac display can have multiple Mac windows within that single display frame. Multiple displays are not possible. A single Mac display and multiple Vision Pro app windows, yes. Multiple Mac displays, no.
Not really. Crazy powerful would be multiple virtual monitors, but that's just not technically possible with 4K resolution and bandwidth limitations. Vision Pro would need a Thunderbolt port and a physically connection to a Mac in order to make that possible. Ultimately this comes down to native apps. If developers just take their iPad apps and tweak the interfaces, that won't be very impressive.