I'm not sure at all but I think you go to change the sms app by default by going to a contact -> Selecting them -> Select send sms -> You should now be prompted to select Handcent or the Default sms -> Tick the box {Lower left of pop up i beleive} & select handcent -> It is now your Default

Make sure all Notifications on the Default sms are off too:

Go to the Default sms app -> Press the "Menu" button -> settings -> Un-tick the "Vibrate" box if on -> Un-tick the Notification box

Hope this helps

Carl