Man, and just last week I had made a comic about MMO inflation. A certain other MMO came up where they are literally able to attach a real-world value to in-game assets due to the ability to buy and sell paid time.
All RO needs now is a permanently destroyable gear mechanic and the circle will be complete!
If they do want to stop a hyperinflation they have to take several steps:
1. Create an reserve ingame currency that is purchased by real life Money (thats done -> Silvervine)
2. Make sure Silvervine cant be used to by ingame items (thats done too its only useable for Cash Shop items)
3. Make sure you cant change Silvervine back for real life money (yeap done)
4. Make Silvervine tradeable between players (yeap done)
5. Let players use Silvervine to purchase VIP (not done, would be a good idea its not entirely a nessecity, but would allow f2p people to get thier hands on vip encouraging them to play more and keep paying people more engaged ... good working example for that is Eve Online with plex)
6. Make Silvervine buyable with (a fixed and by gm adjustable) amount of zeny (a cap should be implemented, a somewhat reasonable amount would be 1/2 of the silvervine needed to buy vip for a month ,as a monthly cap) (not done ... and of course bots need to be dealt to even attempt that)
7. Make sure silvervine cant be changed back to zeny ,except with players (thats done too)
This way you have zeny <> silvervine/vip <> real life money . Zeny is only used for ingame items(or to buy silvervine) , Silvervine is only used for Cash shop items (you should still always implement events like the amistr bag, given f2p the ability to get thier hands on this staff without exchanging to silvervine first) , and real life money is only used for silvervine.
With this you make sure that zeny doesnt loose all of its value, at least its always worth its fixed amount of silvervine that is worth a fixed amount of real life money, this worked on some mmos quite well