Assuming it still gets support that far out, I'd say 35-40 million. I wouldn't expect it to do as low as 20 unless it does so badly that retailers just abandon it after a couple of years.
At the same time, the fast moving smartphone and tablet market will make ANY non-Nintendo gaming handheld an iffy proposition at best. Most other handheld devices will be far above the Vita in performance a few years out, and devs will have only refined and expanded their offerings on those devices in that time frame--making the Vita an even harder sell, killer apps or no.
I was looking back at old charts for the PSP in it's first year, and the sales numbers were insane. You had a bunch of random ass games I don't even remember seeing at the time pulling in >500k, and stuff like GTA pulling in ~1m. With the ridiculous cost of the Vita's memory, and the size of full retail releases, there's no way lack of tracking on download sales can account for that difference.
I love my PSP, but I just don't see the Vita getting anywhere near PSP sales, regardless of how good the library gets. The rapid hardware growth in the handheld market means that dedicated handhelds will have to appeal to a smaller and smaller number of hardcore gamers. Especially if it doesn't have Nintendo's stable of software behind it.
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