I am not sold on the premise that using the 1.12 patch means it isnt Vanilla. If you played from beta or retail release, there was a ton of pain involved. 1.12 was a stability point, both in terms of mechanically, and gameplay. Vanilla fans have been playing on private servers for years, mostly on the 1.12 patch. I have heard almost zero concern about classic being based on 1.12, other than questions about whether certain content is available at launch, or phased in linearly as it was in retail. Pretty much the biggest concern I have heard has to do with Battlenet allowing cross-faction interaction, which in this age of social media and platforms is a fairly moot point.
If the die-hards consider it Vanilla, how isnt it?