Soundwave on 09 June 2015
Both 3DS and Vita got impacted by the smartphone tsunami. However Nintendo's "house" has a better foundation, so their house wasn't as badly damaged, whereas Vita's house got completely destroyed in the storm.
Nintendo still has the kids market, Nintendo loyalists, and the Japanese market to lean on for the 3DS, plus they had a good 1st holiday season for the machine which established an ok working base for the system going forward.
Also taking Monster Hunter away from Sony was like kicking someone in the balls when they're already down. That's the proverbial straw that broke the Vita's back. That's probably the smartest single thing Iwata has done in the last 5 years.
