Open the phone and turn the ringer volume all the way up (using the
bar on the left side).  The highest setting is "Vib & High".

How to get it to only vibrate, rather than vibrate and ring, is
something I've never figured out.  I recorded a silent ringer and
use it that way.

Dennis Ferguson

I had one of those phones, I think it has something to do with the side
buttons. Try holding the side buttons in with the phone closed and see what
the screen does. Hold down two till you see the mode it's in then the upper
button until you change it to what you want then both again to return to
the main screen (phone closed).

manual & even

vibrating, but

I have the same phone, and this kept happening to me a
well.  In my case, the phone's software was a bit out of date an
needed to be reprogrammed.  If in doubt, just take it to the stor
that you bought it from, and they should be able to get it fixed