VIP is an item related to the Mail "Smart Mailbox" or Contacts "Smart List" constructs, in that it is just something that can be used in a search from which the application can do something like show the results in one of its normal views. While the application can use these results internally, it would need to provide a mechanism for a user to access or use them (or not). As mentioned, if a term (such as a VIP property) is not declared an application's scripting dictionary, the app doesn't provide it, and you can't just make it up.
Mail places the VIP data in the VIPMailboxes.plist (XML) and VIPs.plist (binary property list) files in the ~/Library/Mail/V10 (or whatever version) /MailData folder, where they can be used by the app along with other search terms. It may be possible to edit these files, although you would need to figure out what they are using for the keys and how they are linked to the messages.
Topic:
App & System Services
SubTopic:
Automation & Scripting
Tags: