FAST Cache and FAST VP are two different things. Both potentially can improve performance. FAST VP (or sometimes called "Auto-tiering") is always good practice for Pools as it will continually monitor the Pools performance and move the hot data around to the most optimal tier within the Pool. FAST Cache can speed up access for data that has a high locality of reference.
You might want to review the White Papers for both to help you determine which will be the most beneficial, but FAST VP should always be used in my opinion when you have pools as it will re-balance the hot IO within a tier and move hot IO up to the faster tire.