The only thing that I've found is "Settings", "Sound & display", "Haptic feedback"...
It states that "Vibrate when pressing soft keys and on certain UI interactions". So that should turn off the vib when pressing the 4 soft key buttons, but I don't know what else it would turn off.
This does appear to be different that the haptic feedback on the virtual keyboard, which is in another section of "Settings" ("Language & keyboard").
Hope that helps.