Yes, you do need to be VIP+ to access the mirrorclient. Here's a link to the thread that explains the benefits of the client. http://osbot.org/forum/topic/66338-reducing-banning-rates-behind-the-scenes-at-osbot-27-febuary-2015/
It's basically a workaround the client detection system Jagex uses to detect runescape bot clients. So far many people have used it with no bans so far, so I guess it really is working. Also, you can buy VIP by going to the Home page and then clicking PREMIUM SCRIPTS on the top of the screen, then select the option to become VIP or whatever you want on the page that opens, and then you just pay.