This will work, though if you are queasy about using a metal fork, you may also use three credit cards:
Same as with the fork method, install the flange, then pull it and the internal mount down further with your fingers/hands, thus pulling the mount and flange out of the display just enough to be the right height for the side screws to line up properly. Slip 3 credit cards, horizontally and parallel to the flange, into the space created in the top to keep the tension and the extension. Then, put the VESA mount overtop the flange and the credit cards and everything will line up properly. Screw in side screws and center top screw. Slip fingers in the sides and under the installed mount and wiggle out the credit cards - which is fairly easy because the mount now holds the tension. This crucial step is not listed in the adapter instructions and it is absolutely critical. Documentation fail - but thanks for the support community for filling in the all important blank.