Mostly due to smartphones Sony can't do anything to save the Vita sales wise (though it's far from dead for me personally as I often enjoy Minecraft, Sly, Rayman, P4G, Killzone, Spelunky, Resogun, etc.).
However I think Sony could've done some things differently before Vita's launch. Some of the things would've been expensive but I'm sure they would've improved sales a lot.
1. Sony should've bought Level-5 prior to Vita's launch. While not developing many AAA games Level-5 develop a bunch of great AA games with relatively short development times which would've been perfect for the handheld including Dark Cloud + Dark Chronicle, Rogue Galaxy, Yo-Kai Watch, Danball Senki, Jeanne d'Arc, White Knight Chronicles, Ni no Kuni, Professor Layton, Inazuma Eleven, etc. I know it's easy to say in hindsight but imagine every Vita being bundled with Yo-Kai Watch from launch.
2. Sony should've secured more 1st and 3rd party AAA games. Top 10 best selling PSP games include GTA, GT, GOW, FF and Monster Hunter. GT and GOW for Vita would be easy since Sony own the IPs and even though an exclusive GTA, FF and Monster Hunter could be expensive I think it'd be worth the cost (personally I'd love a Diablo/Torchlight port as well). In this way they could've showed potential customers their huge commitment PRE LAUNCH and made it clear that their handheld would offer unique experiences not available on smartphones. This would cater to the huge group of core and casual gamers that bought PS1+PS2+PS3+PS4.
3. Sony should've ensured more attention and visibillity for their Vita in stores similar to the home consoles and Nintendo's handhelds. If every big western store had a blue wall with some Vitas to try surrounded by its 30 or so best retail games (like in big Japanese stores, see picture) I think it would do a lot better. I'm sure a lot of Vita's titles could attract attention from the mainstream customer.
4. No gimmicks. Sony should've provided their handheld with L2 and R2 triggers instead of the back touchpad.
5. No cofusing name. PS Vita should've probaby just been called PSP 2. It's like they went with the codename. PS4 was "Orbis" and PSP 2 was "Vita" (circle of life).
6. Built in storage and cheaper memory cards. Vita should've at least 16GB built in memory and if micro SD support would be the only way to ensure cheaper storage so be it.
7. Sony should get those cheap (but great) smartphone games on their handheld. All the best smartphone games + exclusive and high quliaty AAA and AA games = win. Now I'm NOT talking about shitty (imo) apps like Candy Crush, Flappy Bird, Jetpack Joyride, Temple Run, Angry Birds, Farmville, etc. but rather those few gems like XCOM, World of Goo, Hearthstone, Monument Valley, The Room, Oceanhorn, Dead Space, Mass Effect Infiltrator, Deus Ex: The Fall, COD: Strike Team, Modern Combat, Shadowgun, Sonic CD, GTA 3/Vice City/San Andreas, SW KotOR, etc.
