On the top of my head I would recommend Persona 4 Golden, Freedom Wars, Soul Sacrifice Delta, Gravity Rush, Wipeout, Killzone, Tales of Graces R, Hot Shots Golf, Uncharted and Dynasty Warriors Next. These games pretty much got me through long boring train rides for the past 4 years and are easy to just jump into and continue when you need to.
Disgaea 3 and 4 are pretty hit or miss depending on how you like your games, personally I couldn't really get into them. Hyperdimension Neptunia games which seem to be shitted out every season now can be fun if you're a grinding masochist. As for the ports, Injustice, DOA5, Metal Gear Solid and Marvel vs Capcom 3, Dragon's Crown and Final Fantasy X + X-2 are pretty solid ports, Borderlands 2 ported over fairly well if you don't care that the framerate is like 26fps.
If you're into rythym games DJ Max and the Hatsune Miku games are pretty good. Project Diva F2nd is actually 20 dollars right now and can be addictive if you're into robotic sounding music sung by a robots designed like teenagers and children.
There's no monster hunter for vita, but Ragnarok Odyssey and Toukiden aren't too shabby as replacements.
The ninja gaiden games aren't too bad either.
The weeb games are pretty hit or miss, and I'm too lazy to name them all. If you're into stripping girls you should look into Akiba's Trip. If you're into having forced sex with female classmates and impregnating them to create blessed children to fight for you you should look into Conception 2. If you're into rubbing the breasts of women and recruiting them into your party, I'd recommend Valhalla Knights 3. If you're into semi-naked girls enrolled in a ninja academy fighting each other, I'd recommend Senran Kagura Shinobi Versus.
Oh, and if you're ever under the influence of narcotics, you may find Reality Fighters pretty exciting.
Anyways, if you have a game you're interested in for vita, just ask because I have no life and most likely played it.