VIP server is now open for public, meaning that you don't have to pay to enter it. It won't last forever, it's just for a week, and the only disadvantage is that the account slots are limited, meaning that a maximum of 30 players can enter it at once.
Why is the VIP server better than the others? Because of the following:
-99% success rate in Chaos Machine
-20 golden monsters / every invasion
-drop rate increased
-all events + custom events enabled