I overheard somebody consoling their friend that Monster Hunter would save Vita on my commute today. You guys aren't doing your jobs!
Anyway, dunno if anybody's interested in how Soul Sacrifice will do and its effect on the Vita, but here's some data on similar games.
(Everything from Garaph.)
PSP 8/31/2006 Valhalla Knights 64,506
PSP 5/29/2008 Valhalla Knights 2 89,004
PSP 6/25/2009 Valhalla Knights 2: Battle Stance 33,271
Wii 10/8/2009 Valhalla Knights: Eldar Saga 11,613
PSP 7/31/2008 Phantasy Star Portable 633,954
PSP 10/8/2009 Phantasy Star Portable PSP the Best 40,488
PSP 12/3/2009 Phantasy Star Portable 2 590,527
PSP 8/26/2010 Phantasy Star Portable 2 (PSP The Best) 15,274
PSP 2/24/2011 Phantasy Star Portable 2: Infinity 373,309
DS 10/22/2009 Element Hunter 11,635
DS 12/3/2009 Monster Busters 61,009
DS 1/20/2011 Monster Busters Powered 47,648
PSP 2/4/2010 God Eater 617,828
PSP 10/28/2010 GOD EATER BURST 436,158
PSP 9/15/2011 God Eater: Burst (PSP the Best) 3,602
PSP 10/14/2010 Lord of Arcana 142,388
PSP 12/17/2011 Lord of Apocalypse 17,910
PSV 12/17/2011 Lord of Apocalypse 29,111
Wii 6/23/2011 Earth Seeker 4,327
PSP 8/4/2011 Toriko: Gourmet Survival 138,838
PSP 7/5/2012 Toriko: Gourmet Survival! 2 74,862
Phantasy Star Portable is the largest LTD here, though God Eater is the largest true MH clone.
What I find interesting is that the sequels here have almost universally landed below their predecessor. Only Valhalla Knights 2 increased from its predecessor, but the original was released before Monster Hunter exploded (MHP2). The second game had Monster Hunter style boxart.
It may be that audiences are interested in the idea of a high-quality Monster Hunter alternative but lose interest without the huge MH playerbase. Soul Sacrifice may not be able to generate prolonged interest in the Vita.
Famitsu's reader poll showed Soul Sacrifice above God Eater 2. Even if God Eater 2 drops as much as Burst did and Soul Sacrifice sold the same amount, it would still be the best-selling Vita title. Though, I would expect God Eater to have far more mainstream appeal.