Check Redmine plugin installation procedure.
This plugin has been developped tested on Redmine 2.1-stable branch. It should work on 2.x
Go to Administration -> Users and then select a user.
In Information part below Administrator, you should see a VIP checkbox.
To check if the user is VIP one, you can send it a :vip? message.
author.vip?
assignee.vip?
User.current.vip?
...
Look at my SLA plugin to see it in action.