Vibration coupling through clothes as function of frequency
Haptic feedback actuators are more common now with both linear resonant actuators (magnetic spring mass) as well as eccentric rotating mass vibration motors.

Linear actuators are most efficient and reliable but with lowest strength in mobiles but allow distinct feedback amplitude bursts.

Essentric Rotating mass vibrating motors increase linear frequency with voltage but impact is quadratic with rotational energy and fixed displacement, but heat energy limits frequency of small motors.

There is much more written about vibration isolation methods than actuators thru protection vests.

Old pagers used larger essentric mass vibrating motors but were less reliable, but in this case more detectable.

The frequency response of a bulletproof vest depends on the type, and tension or coupling mount of actuator, but is basically a low pass filter with weak coupling if loose for a small displacement actuator.