this should be pretty easy, however I haven't tried this. But I do have a copy of SystemSoundBehaviour.plist with me so I can make a guess...
Open SystemSoundBehaviour.plist with PropertyList Editor
Find:
Code:
<key>ScreenLocked</key>
<dict>
<key>RingVibrateIgnore,SilentVibrateOff,RingerSwitchOff</key>
<array/>
<key>RingVibrateIgnore,SilentVibrateOn,RingerSwitchOff</key>
<array/>
<key>RingVibrateOff,SilentVibrateIgnore,RingerSwitchOn</key>
<array>
<string>Beep</string>
</array>
<key>RingVibrateOn,SilentVibrateIgnore,RingerSwitchOn</key>
<array>
<string>Beep</string>
</array>
</dict>
Replace with:
Code:
<key>ScreenLocked</key>
<dict>
<key>RingVibrateIgnore,SilentVibrateOff,RingerSwitchOff</key>
<array>
<string>Vibrate</string>
</array>
<key>RingVibrateIgnore,SilentVibrateOn,RingerSwitchOff</key>
<array>
<string>Vibrate</string>
</array>
<key>RingVibrateOff,SilentVibrateIgnore,RingerSwitchOn</key>
<array>
<string>Beep</string>
</array>
<key>RingVibrateOn,SilentVibrateIgnore,RingerSwitchOn</key>
<array>
<string>Beep</string>
</array>
</dict>