The file was called vold.conf in previous android releases, but is now vold.fstab in 3.2
So the command is:
adb pull /etc/vold.fstab vold.fstab
That will copy the file to whatever directory your adb is in. Then you just do the edit and push it back the same way.
There's a little more information in this thread I posted when I was having some trouble:
http://forum.gtvhacker.com/sony-release ... html#p5415