Looks like Last Mission will be included in Final Fantasy X/X-2 HD Remaster
Yesterday, some leaked off-screen images surfaced, showing off the title screens for the PS3 and Vita versions of the game. Most interestingly, these screens showed a selection for Last Mission, the Final Fantasy X-2 post game bonus level included with the original international release.
Today, PlayStation Vita update 2.60 added functionality to easily upload and download your cloud saved data via PlayStation Plus, which is done via the icon added to the game Live Area screen. We see this same icon in one of the leaked screenshots, so it looks like this is the real deal.
Final Fantasy X|X-2 HD Remaster has been announced for release in 2013, but a release date has yet to be revealed.