I don't know about vibrating harder, but you can make it vibrate longer...
Take a look at the following registry key:
HKEY_CURRENT_USER\ControlPanel\Sounds\RingTone0
"Script"="v3w3apw3r"
That tells the phone...
1. (v)ibrate 3 seconds
2. (w)ait 3 seconds
3. (a)ctivate device
4. (p)lay ringer
5. (w)ait 3 seconds
6. (r)epeat
Play with those settings and see what you come up with! Make it vibrate longer, wait less. Good luck, and post what you find out!
edit: this is the setting when you set it to vibrate then ring...