Okay.

apparently there is "Donegal Tweed" and there is "Donegal tweed". A Google search convinced me that what someone told me was a tweed yarn that would sub for the Turnberry is in fact, Takhi's "Donegal Tweed (tm)" and not the more general "donegal tweed" (which seems to be a generic type.)

dang. Back to square one on obtaining yarn for the Bookworm Vest. None of my trusted fiber-suppliers have Turnberry Tweed that I can see, and I've not had any glowing recommendations for the places that have it.

maybe I need to look for a sort-of-aran-weight tweed yarn and go from there.