The Metal Gear Solid HD Collection just made it to the Vita, offering sneaking on the go. But contrary to its name, it isn't quite HD, failing to run at Vita's native 540p resolution, to boot. Astute pixel-counters have noticed a few graphical shortcuts in the game, but it isn't all bad news.
A tech analysis on Eurogamer tells the tale. Vita's native resolution in general isn't HD, to start with, but it is more than SD. While the Vita's native pixel density is 960x544, the game runs at a base resolution 720x448. This makes it higher definition than the original games, in that it now fits the wider aspect ratio, but not really HD. (The original games ran at 512x448.)
On the other hand, some graphical overlays do run at the native resolution. On the whole, the analysis finds that the Vita's progressive scan and OLED improves the look of the game, even despite the resolution issues. For us normal human beings without equipment and eagle-eyes, we'd probably never notice the difference.