Are you sure it's the tweed itself? Many waistcoats have a stiffener sewn in that is quite, well, stiff. This is often bonded to the tweed itself. Compare the arms of the matching jacket (assuming that there is one) to the body of the waistcoat.

Also, not sure what you mean by luxury. Was this a custom-tailored waistcoat in Harris Tweed, made by a gentleman's shop? Some of the highland suppliers title their wares a bit...misleadingly.

A luxury waistcoat: http://www.meyerandmortimer.com/new-page-1