The Vita has no games. There is no Vita. Nothing is real. All our lives are the products of our or someone else's imagination.

That said, I'd recommend the following:

Virtue's Last Reward - the best of the Zero Escape series, with dozens of hours of content
Wipeout 2048 - one of the best and most engaging Wipeout titles ever
Severed - arguably DrinkBox's best effort, this dungeon crawler is filled with new ideas
Gravity Rush - a unique action-adventure game with a lot of personality
Killzone Mercenary - the best Killzone game in a generation, with neat upgrade mechanics
Guacamelee! - a robust and action-packed Metroidvania
Shiren the Wanderer 5 - a punishing rogue-like that will keep you coming back for more
Retro City Rampage - an 8-bit Grand Theft Auto clone with a wicked sense of humor

Of my +20 games, I've only played a hand ful.  But I'm proud of my collection...

Soul Sacrifice definitely gets a big thumbs up from me. Can't stand Monster Hunter games generally, but this game really nailed it for me. And the soundtrack is just incredible!

I think for the most part you've already got the absolute must haves. How on earth is Killzone still Vita exclusive?

On the exclusives front, I'd also give honourable shout outs to Everybody's Golf and WipeOut. Also Lumines: Electronic Symphony, which IMO gets nowhere near enough praise.

There's also quite a few games I think are "best on Vita", particularly the indies because the art style really pops on the screen and they are very suited to handheld gaming such as Hotline Miami.

That's a fun list. I have most of your collection, but have not played many of them. It took me a bit to get into into it, but I LOVED Demon Gaze. I got very into the characters and story. There was a sense of warmth and camaraderie that was palpable. In a way, I didn't want it to end.

Demon Gaze is definitely under-rated. One of the better DRPG's I've played with an oddly enjoyable plot.

Tbh, I feel like quite a few Vita JRPGs flew under the radar. Ar no Surge is another great choice. And Lost Dimension is a criminally ignored game.

Here is my list of my favorite Playstation Vita games (I don't own everyone in this list and I am not including PS1, PSP and Indie games). :)

Minecraft
Tearaway
Killzone: Mercenary
Uncharted: Golden Abyss
Soul Sacrifice (most underrated game on PS Vita imo)
Need for Speed: Most Wanted
Gravity Rush
Persona 4: Golden
Final Fantasy X / X2 HD Remaster
God Eater: Resurrection
God Eater 2: Rage Burst
LittleBigPlanet Vita
Freedom Wars
One Piece: Burning Blood
Assassin's Creed 3: Liberation
Zero Escape: Virtue's Last Reward
Zero Escape: Zero Time Dilemma
Street Fighter x Tekken
Borderlands 2
Dragonball: Battle of Z
Mortal Kombat
Ys: Memories of Celceta
Injustice: God's Among Us
Rayman Origins
Rayman Legends
Toukiden: The Age of Demon's
Toukiden 2
Tales of Hearts
Wipeout 2048
DanganRonpa
DanganRonpa 2
DanganRonpa: Another Episode
Sly Cooper: Thieve's in Time
Sword Art Online: Hollow Fragment
Sword Art Online: Lost Song
God of War Collection
Ratchet & Clank Collection
Jak & Daxter Collection
Sly Cooper Collection
Metal Gear Collection
The Legend of Heroes: Trails of Cold Steel
The Legend of Heroes: Trails of Cold Steel 2

Forgot to mention an early Vita gem. Virtua Tennis 4. 60fps full console quality presentation. Absolute blinder on the go as it's such a pick up and play title.