Alright, I've been thinking about this for awhile now. Lately I've been into Xbox since E3. The announcements they  made were stunning! Backwards compatability (I had a 360), Rare Replay, Gears of War, Fable and Halo 5. I've also wanted to play Conker's Big reunion, Master chief collection and other games I really want to play.   I love my Vita, but I don't play it due to lack of games. For the past year I've been struggling to find games for it, last game I got was a MK psp game 2 months ago. It feels like there's nothing left for the Vita and I know I'll be playing the Xbox One alot more than the Vita... truth be told I think I'm still having the Vita because it looks so beautiful!   

I have Border Lands 2, Little Big Planet, Killzone Mercenary, Jak and Daxter Collection, Mortal Kombat, Uncharted Golden Abyss, Unit 13, Metal Gear Soli HD Collection(Digitial) Mortal Kombat(PSP download), 16 gb memory stick, The Amazing Spider Man (Digital), Playstation All Stars Battle Royal(Digital), MegaMan X4(Digital), GTA Vice City(PSP download) and The Impossible game. I have a Vita hand grip aswell with a Vita case and of course the charger. How much should I sell it for? I am thinking $250 - $300.

and I do own a PS4 and Wii U

Don't sell it, save up till Xmas or Black friday and there will be some great bundle prices. It's only June.

You know, 250 sounds fair. But, ultimately, demand will dictate what it's worth. You have a nice selection of games, so I think your asking price is more than fair, but, the demand may say otherwise. Try ebay, and see what happens. You may end up getting more than you ask for, or less. Goodluck.

I sold mine around year ago for 200$ (750zł in Poland). I think it was right and only choice. Don't take me wrong, I loved this console and games made for it (especially Persona 4 Golden), but lack of interest and support from Sony and 3rd party producers made me do this.
Did I regret this decision? No.
Does it help you? Probably not, coz you must know for yourself if you still have games or time to play on your Vita, but keep in mind this: no more good game will come to Vita.

Yeah, sounds like you should sell it but wait til the holidays to grab a xbox one. There's always better deals for those consoles then. I'm not familiar with the pricing on a used vita but I'm sure the others will give ya good idea on that.