Out of curiosity, I tested this codec with various settings using the attached test files and script: [attachment=7254:fdkaac_vbr.zip]
Available VBR qualities for each <profile, #channels, sample rate> by this encoder are as follows:
LC-AAC 1ch 11250 1
LC-AAC 1ch 12000 1
LC-AAC 1ch 16000 1,2
LC-AAC 1ch 22500 1,2,3
LC-AAC 1ch 24000 1,2,3,4
LC-AAC 1ch 32000 1,2,3,4
LC-AAC 1ch 44100 1,2,3,4,5
LC-AAC 1ch 48000 1,2,3,4,5
LC-AAC 1ch 64000 1,2,3,4,5
LC-AAC 1ch 88200 1,2,3,4,5
LC-AAC 1ch 96000 1,2,3,4,5
LC-AAC 2ch 8000 1
LC-AAC 2ch 11250 1,2
LC-AAC 2ch 12000 1,2
LC-AAC 2ch 16000 1,2,3
LC-AAC 2ch 22500 1,2,3,4
LC-AAC 2ch 24000 1,2,3,4
LC-AAC 2ch 32000 1,2,3,4,5
LC-AAC 2ch 44100 1,2,3,4,5
LC-AAC 2ch 48000 1,2,3,4,5
LC-AAC 2ch 64000 1,2,3,4
LC-AAC 2ch 88200 1,2,3,4
LC-AAC 2ch 96000 1,2,3,4
LC-AAC 3ch 8000 1
LC-AAC 3ch 11250 1,2
LC-AAC 3ch 12000 1,2
LC-AAC 3ch 16000 1,2,3
LC-AAC 3ch 22500 1,2,3,4
LC-AAC 3ch 24000 1,2,3,4
LC-AAC 3ch 32000 1,2,3,4,5
LC-AAC 3ch 44100 1,2,3,4,5
LC-AAC 3ch 48000 1,2,3,4,5
LC-AAC 3ch 64000 1,2,3,4
LC-AAC 3ch 88200 1,2,3,4
LC-AAC 3ch 96000 1,2,3,4
LC-AAC 4ch 8000 1
LC-AAC 4ch 11250 1,2
LC-AAC 4ch 12000 1,2
LC-AAC 4ch 16000 1,2,3
LC-AAC 4ch 22500 1,2,3,4
LC-AAC 4ch 24000 1,2,3,4
LC-AAC 4ch 32000 1,2,3,4,5
LC-AAC 4ch 44100 1,2,3,4,5
LC-AAC 4ch 48000 1,2,3,4,5
LC-AAC 4ch 64000 1,2,3,4
LC-AAC 4ch 88200 1,2,3,4
LC-AAC 4ch 96000 1,2,3,4
LC-AAC 5ch 8000 1
LC-AAC 5ch 11250 1,2
LC-AAC 5ch 12000 1,2
LC-AAC 5ch 16000 1,2,3
LC-AAC 5ch 22500 1,2,3,4
LC-AAC 5ch 24000 1,2,3,4
LC-AAC 5ch 32000 1,2,3,4,5
LC-AAC 5ch 44100 1,2,3,4,5
LC-AAC 5ch 48000 1,2,3,4,5
LC-AAC 5ch 64000 1,2,3,4
LC-AAC 5ch 88200 1,2,3,4
LC-AAC 5ch 96000 1,2,3,4
LC-AAC 6ch 8000 1
LC-AAC 6ch 11250 1,2
LC-AAC 6ch 12000 1,2
LC-AAC 6ch 16000 1,2,3
LC-AAC 6ch 22500 1,2,3,4
LC-AAC 6ch 24000 1,2,3,4
LC-AAC 6ch 32000 1,2,3,4,5
LC-AAC 6ch 44100 1,2,3,4,5
LC-AAC 6ch 48000 1,2,3,4,5
LC-AAC 6ch 64000 1,2,3,4
LC-AAC 6ch 88200 1,2,3,4
LC-AAC 6ch 96000 1,2,3,4
HE-AAC 1ch 22500 1
HE-AAC 1ch 24000 1
HE-AAC 1ch 32000 1,2
HE-AAC 1ch 44100 1,2,3
HE-AAC 1ch 48000 1,2,3
HE-AAC 1ch 64000 1,2,3,4
HE-AAC 1ch 88200 1,2,3,4,5
HE-AAC 1ch 96000 1,2,3,4,5
HE-AAC 2ch 16000 1
HE-AAC 2ch 22500 1,2
HE-AAC 2ch 24000 1,2
HE-AAC 2ch 32000 1,2,3
HE-AAC 2ch 44100 1,2,3,4
HE-AAC 2ch 48000 1,2,3,4
HE-AAC 2ch 64000 1,2,3,4,5
HE-AAC 2ch 88200 1,2,3,4,5
HE-AAC 2ch 96000 1,2,3,4,5
HE-AAC 3ch 16000 1
HE-AAC 3ch 22500 1,2
HE-AAC 3ch 24000 1,2
HE-AAC 3ch 32000 1,2,3
HE-AAC 3ch 44100 1,2,3
HE-AAC 3ch 48000 1,2,3
HE-AAC 3ch 64000 1,2,3,4,5
HE-AAC 3ch 88200 1,4,5
HE-AAC 3ch 96000 4,5
HE-AAC 4ch 16000 1
HE-AAC 4ch 22500 1,2
HE-AAC 4ch 24000 1,2
HE-AAC 4ch 32000 1,2,3
HE-AAC 4ch 44100 1,2,3
HE-AAC 4ch 48000 1,2,3
HE-AAC 4ch 64000 1,2,3,4,5
HE-AAC 4ch 88200 1,3,4,5
HE-AAC 4ch 96000 3,4,5
HE-AAC 5ch 16000 1
HE-AAC 5ch 22500 1,2
HE-AAC 5ch 24000 1,2
HE-AAC 5ch 32000 1,2,3
HE-AAC 5ch 44100 1,2,3
HE-AAC 5ch 48000 1,2,3
HE-AAC 5ch 64000 1,2,3,4,5
HE-AAC 5ch 88200 1,3,4,5
HE-AAC 5ch 96000 3,4,5
HE-AAC 6ch 16000 1
HE-AAC 6ch 22500 1,2
HE-AAC 6ch 24000 1,2
HE-AAC 6ch 32000 1,2,3
HE-AAC 6ch 44100 1,2,3
HE-AAC 6ch 48000 1,2,3
HE-AAC 6ch 64000 1,2,3,4,5
HE-AAC 6ch 88200 1,4,5
HE-AAC 6ch 96000 4,5
HE-AACv2 2ch 32000 1
HE-AACv2 2ch 44100 1,2
HE-AACv2 2ch 48000 1,2
HE-AACv2 2ch 64000 1,2,3
HE-AACv2 2ch 88200 1,2,3,4
HE-AACv2 2ch 96000 1,2,3,4