I was curious about that and found this video while browsing GameFAQ's:
https://www.youtube.com/watch?v=EkQtUwXhm-0&feature=youtube_gdata_player
Basically:
1) Make a party on the vita (it could just be you in the party)
2) Start the beta and in the multiplayer menu, choose "party match"
3) You'll be in an empty lobby. Just choose "launch game".
(If you notice, the game mode will say "Mercenary Warfare").
That's it! I just tried it and it does work. I got into a game with 3 other people. Mercenary Warfare = Free for All.
You may not get into a full room. The most I've seen is the 3 other people. Let's see what happened when I keep trying. If someone on your friend's list is in a free for all room, they can join you. I just tried this with fellow GAF'er, Drizzt