How to properly compress FMOD vorbis audio

[wareck]

Hello
I follow this guide but I've got some but I'm having trouble finalizing the project...

What I've done:
file.bank => extract FSB file with quickbms
PythonFSB5=>extract all .ogg file
bincat to concatenate all .ogg file

now i do not understand that :
if I use oggvorb2fsb on this file I have a very small gain durring compression almost nothing
If i use oggre I have a very high gain , but crc error when I unpack file...

for now if I do this
file.bank => extract FSB file with quickbms
PythonFSB5=> extract all .ogg files
bincat => one big file
oggvorb2fsb5 => one big encoded file
hdiffpatch between original.bank and my new file
compress encoded file + diff file

during unpack
hpatchz encoded + diff file => restore my original file.bank

It works but very small ratio gain in conversion...

thanks for your help

regards