From vibrate to silent

[Runner310]

In the past two weeks, my Incredible seems to switch periodically from vibrate to silent. I don't think I'm doing anything to do it, but is there a way to ensure the phone is always on vibrate?

 

[meta droid]

do you forget to turn the screen off when you put it in your pocket? because if you hit the side rocker button when the screen is on it will change it between silent, ring, and vibe. other then that its prob a defect.

 

[rogergo4]

Sorry, but which is the side rocker button. I seem to have only volume buttions