Remake your .caf file (there are actually 6 SMS_received.caf files on the phone) at a lower volume, then replace the original volume .caf file with lower volume.caf file.
Only way I know.
You have to be jailbroken and have OpenSSH istalled thru Cydia, then use SSH program on your 'puter.
Or you could use iFile to download and upload the files. Works in any web browser over your local network, and doesn't require SSH installed or an SSH client on your computer.
Remake your .caf file (there are actually 6 SMS_received.caf files on the phone) at a lower volume, then replace the original volume .caf file with lower volume.caf file.
I've tried to edit my .caf files for my received SMS tones (i.e making a silent tone), but once i put them back into the phone through SSH, the vibration is also shut off.