In the last game I played which happened to be one of the most successful MMORTS on facebook called Mobsters by a company named Playdom, they offered an option to become a VIP member with exclusive rewards. You got equipment that was golden and no one else could get, defense/offense bonuses were slightly better. The subscription was 19.99 per month. Not a lot of money for your ROI.
The issue here is that Plarium is choosing who should be VIP setting up a barrier between their customers and themselves, another nail in the coffin so to speak. Marketing to their entire customer base would have worked much better, let the end user or player choose whether or not they WANT to be a VIP by offering a monthly subscription that was within an affordable range for everyone.
Obviously this game doesn't have multiple new weapons they can release etc etc but unique packages could have been offered. Make your player's feel wanted not put up more walls. People feel excluded not included, every major product corporation offers the "vip" package and let the end user decide whether or not they should invest. You have managed to make your VIP package a sort of elite club on the waterfront that only you decide who enters so that when the peasants drive by they can flip you off everyday or cast ill will towards you and never have anything nice to say.