For those looking to convert their 27" 2012-2015 iMac's to a VESA mount, it can be done using OE parts. I'm only writing this because I never found anything on the internet confirming this worked, I just took the gamble and it worked out great.
You need to order 4 parts (assuming you already have a Torx set):
1.) Mounting Bracket - part# 923-0424
2.) Backing Plate - part# 923-0422
3.) Tongue - (2012) part# 923-0423 - (2013) part# 923-0531 - (2014/2015) part# 923-00084
4.) Adhesive Strip Kit + opening tool
I have a 2012 and ran into no issues - worked perfectly, I make no promises that you'll have a similar experience but, at least to me, it felt very straight forward. I got all the VESA parts from www.thebookyard.com and the adhesive strip kit here on iFixIt.
You will need a 5 point Torx TS15 bit for the pentalobe screws that attach the backing plate to the tongue... technically not the correct bit, but there are only two screws and I felt zero slip from the 5 point TS15.