It wouldn't technically be correct to say PS Vita is dead, especially not in Japan. It's definitely been on life support in the west for a while, though, and Sony's American and European branches announced plans to end GameCard production by "close of fiscal year 2018."
The message was sent to developers and reported on over at Kotaku. The 2018 fiscal year ends for Sony on March 31, 2019, and final purchase orders have to be entered by February 15, 2019. Sony went on to confirm the end of production with Kotaku.
Does this mean you should chuck your Vita in a dumpster and scream dramatically? Not quite yet. It's been relegated to a home for niche localizations lately, but time is now running ever shorter for what was, when all is said and done, a pretty decent little system.