When will my Mac (15inch MacBook Pro Mid 2012 ) become a vintage product?
Generally speaking, based in part on information in the linked support article in your question the vintage designation is a 5 to 7 year window, so the two Mid 2012 15" MacBook Pro models vintage windows are shown below:


The MacBook Pro (15-inch, Mid 2012) was discontinued in October 2013. So October 2018 would be the start of the 5 year window, with October 2020 being the 7 year window.
The MacBook Pro (Retina, 15-inch, Mid 2012) was discontinued in Feburary 2013. So Feburary 2018 would be the start of the 5 year window, with Feburary 2020 being the 7 year window.


That said, if you live in Turkey, California or France, make note of what it said in the article, quoted below.

From Vintage and obsolete products:


  Vintage products are those that have not been manufactured for more than 5 and less than 7 years ago. Apple has discontinued hardware
  service for vintage products with the following exceptions:
  
  
  Mac products purchased in the country of Turkey. Owners of vintage Mac products may obtain service and parts from Apple service
  providers within the country of Turkey.
  Products purchased in the state of California, United States, as required by statute
  
  
  Owners of vintage Mac products may obtain service and parts from Apple service providers within the state of California, United
  States. 
  Owners of vintage iPod products in the state of California may obtain service from Apple Retail Stores or by contacting AppleCare at
  1-800-APL-CARE.
  Owners of vintage iPhone products in the state of California may obtain service from Apple Retail Stores or by contacting AppleCare at
  1-800-APL-CARE. For products purchased in France, see Statutory
  Warranties of Seller and Spare Parts.