VN's CEO was quoted as saying that the new 789 and 359 deliveries will have WiFi already installed from the factory, with expansion of the service "on a wide scale" possible down the road. He also mentioned that WiFi may be free on domestic routes.

Would I pay US$15-20? Maybe on a long-haul... I wouldn't pay on flights <6 hours.

My blog entry:
Vietnam Airlines: In-Flight Wi-Fi to be Available on New 787, A350 Aircraft
Disclosure per FT rules: The link above goes to a blog, VNAFlyer, to which I am a regular contributor.