Uncharted was £45 on release and Assassin's Creed £40 so they have actually dropped by half or more since release. I get what you mean, but Vita games don't sell anywhere near the volumes of the home console blockbusters so stores don't have anywhere near as much "overstock".
For the most part I've been able to find any Vita game I've wanted below £20 within a couple of months if I've not felt it's warranted full price with the exception of Call of Duty. However most places that do reduce them tend to then sell out so they don't stay low very long. I got Silent Hill a few weeks after release for £18.85. Amazon's Vita prices are a bit of a scam right now mind. Basically it looks like Amazon are only getting a few copies in, selling out fairly fast then it's just other vendors wanting above RRP for them meaning stuff like Gravity Rush is frequently £40-45 when it's £15.85 at Shopto.
Btw, Silent Hill is £19.85 on backorder at Shopto right now. Last game I ordered from them on Backorder was Unit 13 which shipped a few days later. So it's not like most places where back order usually turns into an order cancellation notice. They're expecting their next batch of them tomorrow.
http://www.shopto.net/video%20games/psvita/PSPVSI00-silent-hill-book-of-memories-ps-vita
http://www.graingergames.co.uk/games/ps-vita/vt16-uncharted-golden-abyss - Uncharted for £19.99 or £16.49 pre owned.
http://www.graingergames.co.uk/games/ps-vita/vt56-assassins-creed-iii-liberation - Assassin's Creed £19.99 or £16.49 pre owned.