The jar or packet at either of my local stores is quite big.   How long does it stay effective, once opened?   Given Star Anise is quite hard, I'm thinking it may last a while.  But would I be better vacuum sealing half or 2/3?  [edited to correct grammar.  rmjf] 
You don't really need to vacuum seal it. You can store it dry in a sealed bag or container. It will last years. The flavour might deteriorate over time though. 