Alright, so two potential issues, it could be one or the other, or both in my case.
Issue: When you flick the normal/ silent switch up and down, the phone does not display the silent or vibrate picture icon.
Solution: You have a broken flex cable. It's a pain in the butt to replace, but somewhere along the connections the phone has developed a break in the connection.
Issue: Your phone does display the silent or vibrate picture icon when thumbing the normal/ silent switch, but the motor does not seem to work.
A: If it's a bad motor, you can just drop a new one in and tighten down the screws. However, it Could very well be…
B: There's crap jamming the motor from spinning. There's a shiny metal nub, the part that spins on a very tiny axle. Whether it's lint, a stray screw, or perhaps a flex cable poking out where it shouldn't be (as in my case, with a new replacement).
Solution is pretty easy, as you have some play if you don't tighten down both screws for the vibrator motor. Leave them loose, and reposition the motor to allow it space to turn.
C: The contacts on the new/ original motor aren't where they need to be. Bending them down in either case seems to improve contact, but in the case of the replacement motor the two copper connectors were actually duck footed; that is they pointed out, instead of being straight and parallel to one another.
I bent them (gently) and carefully positioned the motor in (B) as the new flex cable around the right side of the headphone jack was jutting into the cavity for the vibrate motor to turn.