According to the PS Blog the Vita allows users to have voice chat, even if playing different games:
Party – Fancy a chat while you’re gaming? Speak with your friends anytime you like with Party. The voice and text chat application allows you to talk to up to eight party members, no matter what you’re doing. Party also allows you to set up and jump in multiplayer games and chat with Friends while you play different games, bringing cross-game chat to PS Vita.
Does no one else find it a bit odd that this service is available on the Vita but not the PS3? Cross-game chat has been the most requested PSN feature for years.