Is this case separate from the above case? I have checked Linux kernel sources (2.4.13 only) and I have found no reason why chmod should fail on read-write mounted VFAT filesystem. Perhaps there is some obscure bug/feature in Linux VFAT driver that escapes my eyes. Could you provide more information about this case, please?
Well, it's easy : it depends of the user you mount your VFAT partition with. Look at this example (on ext2) :
nexus6:~$ ls -l toto -rwxrwxrwx 1 ulmer users 0 Jan 29 20:53 toto nexus6:~$ su otheruser Password: nexus6:/home/ulmer$ ls -l toto -rwxrwxrwx 1 ulmer users 0 Jan 29 20:53 toto nexus6:/home/ulmer$ chmod o-x toto chmod: toto: Operation not permitted
So if like me, you had your VFAT partition mounted R/W for everybody on the system (or for people from a particular group) but with owner = root, you will have the chmod problem (I was bitten by it once while playing Baldur's Gate).
This bug is really painful for multi-user systems... In my case, I fixed it to have the VFAT files mounted with me as the owner :-)
Lionel