Directional haptic bracelet
Interesting idea. I take it that the idea is that the bracelet needs to be parallel to the ground (i.e., the forearm needs to be vertical).

How about a different usage model: Have the forearm be horizontal (as if the user is pointing somewhere), and pulse the (single) vibrator using Morse Code to indicate the direction the user is pointing?

Obviously, I'm assuming the magnetometer is mounted on the bracelet itself. If it's mounted elsewhere, other usage models may be possible.