Some more information:
1. Maximum ATRAC9 bitrate supported by PS Vita is 192kbps.
2. By default, ATRAC9 have limited band (all frequencies higher than ~18kHz are cut). To bypass that and achive highest sound quality possible, use -nbands 18 when encoding.
Quality and performance comparision:
1. MP3 320kbps: cuts most of the audio information higher than 16kHz, file size 9.72Mb, core load during playback ~50%
2. ATRAC9 192kbps -nbands 18: keeps everything up to 24kHz intact, file size 5.83Mb, core load during playback ~40%