Thank you red_menace!
I fully agree: it seems to be not described in the dictionary and so not exposed to AppleScript. However, it didn't find any description anywhere at published sites of Apple.
Question to moderators
Can one of the operators give a hint where the VIP implementation architecture or their objects are described? If necessary, I would switch the programming language to one which is able to access these objects. But which language / environment is it?
Comment to manipulating .plist files
I think this is not a good idea because they seem to be just local caches (since the VIP management goes across devices, including iOS and iPadOS). So these objects are not trivial. I can imagine, they are local objects which send messages to the cloud. Anything else doesn't make sense on long term.
Additional critical question to Apple
Why aren't these objects published in the AppleScript dictionary?
It doesn't make sense to have an interface in Apple Mail Rules to be able to call an AppleScript if such a basic object is not exposed. Do you see it differently?