Vibration mode is BlackBerry Curve 8520 smartphone can be turned off using the Sound and Ring tones Option same as changing ring tone in a BlackBerry Curve.
Press menu button on your BlackBerry Smartphone and select Options from the menu list.
In the Options screen navigate to Sounds and Ring Tones Options
In the Phone Loud screen set the value for Vibration to Off. This would switch off the Vibration alert in a BlackBerry Curve Smartphone.