thank you Pmotz -- you are certainly correct.
Given the size of the replacement feet it is almost certainly the case that more than screwing off the coned feet is required. I do indeed have a rubber isolator on the plinth containing a down-facing threaded stud to receive the original cones. It would seem these must come out too to make room but that is far more than is explained in the literature (see: http://www.elusivedisc.com/prodinfo.asp?number=HW-B4008, where it shows the foot and the adaptor). I did tug a little at these but did not want to tear them out (using either method you describe) without some confirmation that this was what VPI intended (they provide NO instructions) or that unscrewing this stud would leave a suitable threaded receptacle on the other side for the replacement feet.
As one looks at the underside of the plinth, there is no way of knowing what wrenching the existing stud out will reveal on the other side. If it 'is' just a threaded receptacle for the new foot, one then wonders what possible role the advertised adaptor plays since it is large enough to cover the replacement foot's stud completely (i.e., one could not screw the new foot onto anything else when the adaptor is in place). Perhaps the removal of the existing stud, as per one of your methods, reveals a larger receptacle into which the adaptor then fits? Who knows? the dealer certainly doesn't and VPI don't care to provide that info to either the dealer or the customer in the form of instructions. Sometimes it's just frustrating being a customer willing to spend $$ on specialist hifi. the time alone on this has killed the joy of the supposed upgrade.
I'll let you all know when I learn anything more from the various parties involved.