Hi!
  OK, so I have the 1 vs 1 conflict rules set and tested. Its a tweak on Otherkind. It works the way I want it to and the numbers are solid. to summarize:
  Each char has a stat for Ambition, Cunning and Vigilance. When there is a conflict, players set their goals (intentions) and then roll three dice each. Then after the roll, they assign one dice each to Ambition, Cunning and Vigilance. Then each player compares Ambition Dice + Ambition Stat vs each other. the higher of the two gets points towards their goal. Also, one player adds Cunning stat to their Cunning die and compares against their opponents Vigilance Die + Stat. If the Cunning total is higher, the difference is the Harm inflicted. Skills let you add +1 to a die, powers give you a bonus die.

  The trick is, this works great. But it is sort of hard wired for one on one conflict.

  So, there are a couple of ways I can handle one vs many:
1) Single player rolls once and compares against many
Pros: Feels like the players can use team work. Less admin.
Cons: Seems like good/bad rolls would have too dramatic of an affect.

2) Single player rolls separately against each of many players:
Pros: Feels like the is too powerful. One bad/good roll doesn;t skew the whole fight
Cons: more dice/numbers/admin.

3) Single player rolls once, one of the many players roll and the rest give bonuses to them.

  So, this game is supposed to be heroic steam punk with Vampires and a hardcore dystopic setting. Typically, one Vampire will be facing down a group of PCs. Which one of these do you think sets the tone best. My guts says to go with #3, is there something you see that makes that a less than optimal idea?

  This isn't really an opinion poll, I am trying to weigh the alternatives and come up with a solution that best matches the setting/genre. I am trying to brain storm.

I like option 1 myself for the fact that it keeps all the players active and it could be more tactical.

If the vampires are more powerful that the players then rather than balancing the two side by giving the vamps higher stats, you could give them abilities that let the vampires reroll low dice. That would fix some of your low-rolling problems anyway.

Brain,
 Thanks for the brainstorming.
  OK, I actually listed these in the order that I thought of the. So, this is the multi-player mechanic that I have had the most time to think about.

  Low rolls are an issue, yes. But high rolls I think are a bigger issue. If I get a really high value on the "one character side" of a conflict, it sets up a situation where that one aspect (ambition, cunning or vigilance) is unassailable regardless of strategy, tactics, teamwork, whatever.

  Also, if everyone rolls average values on dice, it sort of mitigates the effect of teamwork. Unless the entire team is concentrated (and able) to overcome one value, then it amplifies the effect of teamwork. I am not sure if that is what I want...

  I do see what you mean, it's more fun if everyone gets to roll. I'll have to think about it more, what are your thoughts so far?

All,
 OK, the mechanic has to cover 4 combinations:
1) 1 vs 1 - Done, tested, good
2) 1 NPC vs Many PCs - My original question
3) 1 PC vs Many NPCs
4) Many NPCs vs Many PCs

  My design goals are (in no particular order):
a) Simple system, not too complex
b) Short conflicts (3-5 Rounds)
c) Reward Teamwork
d) Prevent no-win situations
e) Reward Strategy and Tactics (I have this on 1 vs 1 and I want that to carry forward)
f) Provide flexibility for the players

  The only problems I have with Option 1, really, are:
1) Extremely high/low rolls can break b and d
2) Can break F if the player on the "one character" side of a one vs many character conflict does not want to harm all the opposing characters equally.

  Also, what Goal is each player rolling for in a multilayer conflict? And does the player on the "one character" side of a conflict get to create a separate goal for each character they are rolling against, or do they just have one overarching goal?

All of the below addresses team of players vs one or more opponents.

Brainstorm:

Let the players declare contingent per-action goals as a group.  3-player example: "Trip the vampire.  If trip successful, pin the vampire.  If pin successful, stake the vampire."  Then everyone rolls and assigns dice.  This could be done 2 ways:

1) players assign goals pre-roll (player 1 trips, player 2 pins, player 3 stakes)
2) players assign goals post-roll (highest roller might pick Trip, with worse rollers opting to just deal and avoid damage)

Each successful action gets you one point closer to achieving your overall conflict goal.  Number of victories required for success depends on the vampire.  So, a 3-step plan culminating in "stake it!" (assuming staking kills and killing is the goal) would be an appropriate plan for a round (or "turn") in which only 3 more points are required.

The opponent(s) get one set of rolls per player character they oppose.  So, in this example, that's 3 sets of rolls, whether it's one badguy, or three, or seven.

I'm a little fuzzy on dealing and preventing damage, but it might be cool if preventing damage was tranferable among players.  Let's say I'm the player with the second action of a round.  I assign high dice and cool powers etc. to produce a very effective "don't get hurt" number.  Seeing that my buddy who goes next is near death, I opt to take the hit and pass the damage-prevention on to him (narrate jumping in the way or some such).  Planning for these contingencies would be fun in both the goal-assigning and dice-assigning phases.

Hopefully this just applies the player thought process from the 1-on-1 to a team setting.  The core is still trading off goal success vs injuries, you just have some cool options about picking who pushes the goal forward and who gets hurt.  There are a lot of player options for maximizing effectiveness, so you'd probably want to throw them opponents with abilities that interfere with smooth execution.  Examples: curse attacks that can't be blocked, mind-reading that allows peeks at player dice before assigning own dice, once-per-round nasty attack, feint.

Hope this gives you some ideas...

Ps,
-David