But I have to get the VIP to create more then 3 characters right?
Maybe that is why they don't want to increase the zeny limit
System overhaul would be required to change the Zeny cap, that's on kRO and definitely would take time.
Unlocking character slots using VIP benefits is a one time thing. You will still have access to the characters even if the VIP time is not running.
Keep in mind, the character slot will become inactive if the character is deleted, considering VIP is not running and the slot is VIP only.
Should it be the case, you will need to get VIP again to create a character on a given VIP slot.