You can change vibration patterns by editing /etc/mce/mce.ini. At the bottom is the line PatternChatAndEmail and a description of the numbers just above. You could try setting the OnPeriod to 0 to disable it, but I never tried it so don't know if it would work. I have edited my mce.ini to change the Policy to only vibrate when the display is turned off. The default was to always vibrate even as I was reading the messages, which was annoying.