Whether you have a PSP or a Vita, you're good to go for ad-hoc multiplayer.
If you're planning on getting a Vita but your friends are stuck in the PSP era, never fear. Sony realizes that since Vita will have access to the same downloadable library as the PSP, there will be a lot of crossover between games. As a result, it's designing the Vita with the capability to engage in ad-hoc multiplayer with both Vita and PSP systems.
Ad-hoc multiplayer games like the Monster Hunter series generally aren't as big in the West as they are in Japan. Even so, Western PSPs have a fair selection of local multiplayer games, with series ranging from Star Wars to Ratchet & Clank. Not every downloadable game with have Vita/PSP support, however, and Sony has not indicated whether UMD games on PSP will work in multiplayer with downloaded games on Vita.
Sony has confirmed the feature for Japan, although this does not necessarily mean that it will see a Western release. Still, unless the feature ends up costing Sony more money than it's worth, Western Vita/PSP pairs should be able to enjoy cooperative Killzone: Liberation just like their Japanese counterparts.