Last major game to be announced was Tearaway, back in August.
Over My Dead Body was in October, but I suppose that might not fit you definition of major releases. Anyway, for a different take on this discussion, I've put together a list of when various post launch Sony published titles were announced for the Vita (with two dates for those who had limited early reveals). I've split them up in quick 'n dirty "big" and "not so big" categories based on "fan appeal":
"Big"
Over My Dead Body - Tokyo Game Show '12
Tearaway - Gamescom '12
Ratchet & Clank: Full Frontal Assault* - Gamescom '12
PlayStation All-Stars Battle Royale* - E3 '12
Sly Cooper Thieves in Time* - GTTV Episode May '12
Soul Sacrifice - Famitsu April '12 / Reveal Event May '12
Gravity Rush - E3 '11
Warrior's Lair - E3 '11
Killzone Mecenary - NGP Reveal / Gamescom '12
Resistance: Burning Skies - NGP Reveal
LittleBigPlanet Vita - NGP Reveal
(Titles with asterisks were were announced earlier as PS3 exclusives.)
"Not so big"
Invizimals: The Alliance - Blog Post April '13
Table Mini Golf - Blog Post March '13
Epic Mickey 2 - Blog Post April '13
Jak & Daxter - Blog Post April '13
Uncharted: Fight for Fortune - Blog Post November '12
Box! Open Me - Tokyo Game Show '12
Table Ice Hockey - Blog Post Sep '12
Smart As... - Blog Post May '12
When Vikings Attack - Blog Post May '12
Travel Bug - Blog Post May '12
Ecolibrium - Blog Post May '12
Sound Shapes - Blog Post Jun '11
(Corrections are welcome if I got any of them wrong)
So, yeah, I'm going to say that there's a clear pattern here of Sony having a preference for announcing big stuff at conferences. Given that those haven't happened yet this year, I don't think pushing the too long since last major announcement argument again and again is particularly productive.