You don't even have to spend anything. Basically, you have some daily quests which can fill the VIP bar (which is also filled up when you buy diamond)
However, it's not really that quick (~3 days for VIP 2, 1 month for VIP 3, 3 months for VIP 4, the eternity for more), and it require a good amount of playing each day (at least one hour, maybe more). And we have no clues on if it will be kept on the Nutaku version.