The difference between the VITA and the PSP is that the PSP had it's own market and wasn't really a competitor to the DS, Nintendo went for a different market with the DS (it wasn't even a GBA successor at first!) and the PSP had an underserved market created by the GBA; so it got games like KH, MGS, MH, etc etc. It didn't even matter that the PSP launched a year later, because the handhelds weren't really competing against each other.
I think that the Smartphone and Tablet penetration in the market that the DS had, made Nintendo go back to the old market, you can see that Nintendogs 3DS wasn't a success like Nintendogs DS was, because the 3DS is not designed for that market, and that market went to look for it's gaming experiences elsewhere.
The problem with VITA is that Sony misread the situation and made a handheld that has very similar performance to the 3DS (bu bu but Uncharted) but failed to grab the 3rd party games (KH, RE, MGS and MH are all going to 3DS), part due to Sony's overconfidence and part because of launching late.
It doesn't help that there is virtually no design behind the VITA, it's a PSP with another analog stick (real analog sticks this time) and a last minute addition of a back touchscreen that isn't convincing anyone, it doesn't work as a R/L 2/3 replacement, and it ends being a useless gimmick (besides Little Deviants, I haven't seen anything interesting from that screen). There is nothing new or groundbreaking about the VITA, it is a boring handheld.
To me it's doomed, and I'm sorry to enter the Doom Train, but the fact that Nintendo went after the same market as the PSP and DS, and the fact that Sony did not secure relevant exclusives, doesn't have hardware selling games (GT Portable is not going to sell the handheld), is letting B teams do the games (bu bu but Bend) instead of their best teams makes the handheld effectively irrelevant.
The only salvation it has, as hardware, is to get hacked to the max, because it just doesn't have the software to be a real competitor in the handheld space. I see another NGP (now I know why it had this codename!) here, and I expect it to be completely irrelevant as any other non Nintendo and non PSP handheld before it.