I have the same problem with my Samsung Droid Charge. I have found this same problem on many websites, but I have yet to find someone with a solution.
With that being said, I have a theory that may have worked for me and would like to see if someone else can try it with success.
I went into the message settings and made sure that the Vibrate setting was checked. But I then went into the select ringtone setting and chose a ringtone other than the default ringtone. My theory is that when the ringtone is set to the default ringtone then the SMS program just sends the notification to the system to be handled. However, if the system sound is turned off then the system just knows to vibrate for calls... everything else it ignores. But if you set the SMS ringtone to a unique ringtone then if the system sound is turned off then the SMS program knows to vibrate when it receives a text.
This is working when I send myself a text, but it really needs further testing. Please reply if it works!!
