This news is a little old. On Friday Sega Japan began streaming some Vita trailers for Phantasy Star Online 2. These trailers show that the Vita version can support up to 12 players at one time for co-op play. This is pretty cool considering most PSP co-op games couldn't support more than 4:
"Sega released the game on the PS Vita in Japan on Thursday. The software download is free, and the game is free to play, although some in-game items have fees. There is also a Phantasy Star Online 2 Special Package sold with exclusive item codes released on the same day for 5,229 yen (about US$58).
Sega launched Phantasy Star Online 2 for Windows last summer. Sega will also release the game for iOS/Android smartphone devices, but the release has been delayed until this year to raise product quality. The game is free-to-play across all platforms."