In DDO the VIP's are NOT able to buy any of the quest packs that they already have access to as VIP. This means that when those adventure packs go on sale the VIP's cannot buy them in preparation of dropping their subscription and becoming Premium. The store tells them "You cannot buy this content you already have access to" or something similar.
This essentially forces VIP's to stay VIP since if they cancel their sub, by the time their sub expires and they CAN purchase the content it is no longer on sale and they'll have to pay full price for it or go back to the VIP subscription. This situation HAS upset many VIP'ers who intend to go Premium some time in the future.
Yes, it works like DDO; if you're a VIP, you can't buy content that you already have access to. If you're planning to drop your subscription at some point, bank your points until then and then wait for things to go on sale again if you want them at a discount! [Patience]