After a recent Android update, my toggle keys for the Nexus 4 will not let me put the phone into vibrate only mode. I can adjust the volume down to it's lowest setting, but it still makes noise for notifications. The phone will vibrate for incoming calls, but the vibrate icon will not appear. I need to see that icon so I don't have to worry that I might have left my volume setting on when my phone is in the office. Any suggestions?
Try holding the volume-down button as you do now, and then after the volume slider reaches the bottom, tap the volume-down button once more. This goes into silent (not vibrate) mode on my Nexus 7 running 4.3, but I think that's because it can't vibrate.
Alternatively, hold down the power button as if you're going to turn the device off. The power-off menu has on-screen buttons for noisy, vibrate, and silent modes.