It's not possible using the power button long press for the Device Options menu in 4.0.4. It always reverts back to "Vibrate" when changing Silent mode. 

**Conclusion**

Wait for a version update above 4.0.4 and hope they fix this BS. (Also, I friggin' hate people who think their phone is silent during meetings or seminars, and their phone just keeps buzzing and buzzing and buzzing. It's #*@!ing Android trolling!)

The only hack I've found is going to Setting > Sound > Vibration intensity and setting ringtone vibration to zero. However, I'm pretty sure that turns off vibration globally, and you'll still see that annoying icon for "silence with vibration" when you use Silent mode trough the power button. :(

**Solution**

Flip down the curtain of the notificaions menu, then double tap the sound icon. That will bring you from sound to vibrate, and finally to mute. Or get a widget for muting.