Well, he did say it's a 3rd party publisher. That would mean:
-EA
-Ubisoft
-Take-Two
-Zenimax/Bethesda
-Sega
-Capcom
-Square Enix
-Kotch/Deep Silver
-Namco Bandai
-Warner Bros.
To list the big ones and technically, these guys have published and are noteworthy IMO:
-CDProjekt Red
-Valve
-Crytek
Now, of all of these, I'd cross out Kotch, EA, Crytek, Bethesda, and CDProjekt based on their to-date, minimal efforts on handhelds. Ditto for Valve, as much as I wish it were otherwise.
So that leaves:
-Ubisoft
-Take-Two
-Sega
-Capcom
-Square Enix
-Namco Bandai
-Warner Bros.
Now, Randy Pitchford saying just about anything means all of nothing nowadays, but he has been vocal about wanting Borderlands on Vita, if Sony foots the bill. Ken Levine likewise, although his word holds a lot more word than Pitchford's, IMO. Take-Two publishes both of these franchises, so it'd be at least worth keeping Take-Two as an option. Not to mention GTA's presence on the PSP.
Of the Japanese publishers, the only ones that really have anything big that are getting the kind of hype we're seeing would be Square and Namco Bandai(as in, Souls). Capcom has said no to Monster Hunter, and outside of that their current franchise selection is mostly fighters. Sega has PSO2, but that doesn't seem like the kind of game that would have someone in SCEE jumping in glee, does it? And aside from that, it's already been announced; if this is the big announcement, it'd be a massive disappointment as far as hyping goes; we knew it's supposed to be coming.
Disclaimer: I want to pick up PSO2, do not jump me.
Meanwhile, Warner Bros. has a fairly limited line-up of games, and we know there's a Batman Metroidvania coming to the PS Vita already, so I'm not sure what else they can add. Ubisoft on the other hand has stated loudly that they want to do more with the Vita. So they're fairly likely. That would make the runner-ups:
-Ubisoft
-Take-Two
-Square
-Namco-Bandai(by a thread, assuming they can get From Software to make a follow-up to Demons Souls)
^This is all speculation based on two minute's worth of thinking.