If your shops are setup similar to official server, in the initial stage of the game zen is what drives the player economy because players need to be able to buy lots of potions and +3+L+Opt Dragon/Legendary/Guardian sets from NPC. Also if you have Moss, players can farm excellent items and items to use towards making Chaos Weapons and Wings from that. On such a low rate server, 50% exp boost [2 + (.5*2) = 3] is huge - for perspective leveling from 200 to 300 may take lets say 90 days, but with VIP they will get there in 60.
It's good that you're not enabling your Pay2Win donation system until after a character reaches level 400; but endgame, as you know, is not dependent on that - it comes down to taking and holding the Castle which is hugely dependent on both the amount of zen a guild can farm and the Master Levels of characters in the guild. Not to mention, a guild alliance which is farming zen at 6x the rate will be able to provide startup zen to new players as a perk for joining the alliance, putting it even further ahead. Once that alliance gains control of the castle they can enter Land of Trials and use Lord Mix giving them access to the best gear in the game. It will be nearly impossible for any competing alliance to win unless all of their members purchase Gold VIP as well.
How much are you going to charge for 1 month of Gold VIP?