All,
  Here is the basic mechanics for setting the scene. The reason I have it set up like this is that I want players to be able to call for a Research Scene in order to get new spells/gadgets before the big fight. This sort of gives them a chance to prepare and power up before a confrontation they are planning for. Also, the GM can still introduce unexpected conflicts, even in non-conflict scenes. Basically, I wanted to give the players a means to say, "this is what I want" and let the GM include or ignore that as necessary.

The procedure is as follows:
  Once the characters are made, players (including the Mastermind) take turns narrating what their characters do, while the Mastermind describes how the world of Steampunk Crescendo responds to their actions.

  Setting scenes is accomplished by taking turns. Each player (Ending with the Mastermind) takes turns setting a scene. They should include the following information before the scene starts:
1) What kind of scene is it?
a) Exploration - The purpose of the scene is to explain the relationship between character(s) and the rest of the setting.
b) Exposition -The purpose of this scene is to understand the characters' thoughts, motivations and relationships better.
c) Manifest - The purpose of the scene is to explain how a character got a new vampire Power (and Weakness). The player will, gain a new Power and a Weakness (of the same cost). At the end of the scene, apply a +1 Corruption/-1 Purity (never above +3 Corruption/-3 Purity). Characters can manifest powers like this during any scene (once per character per scene).
d) Research - The character is developing a Ritual or Gadget.
e) Planning - The character(s) have an objective and this scene is supposed to show how the character(s) are preparing to accomplish this objective.
f) Confrontation - The character(s) have identified the antagonist and are meeting them face-to-face.
2) Where is the next scene set?
3) When is the scene set?
4) What their character is trying to accomplish (just an overview, the details will emerge during the scene)?

  Once that is established, the other players can decide if their character is in the scene.

  Then the Mastermind can describe the initial scene including Goons and Antagonists. And the players can describe how their characters react to the new situation.

  What is really tripping me up is #4.
  I want the players to signal what it is they want from the scene. But I don't want the whole scene to be played out before it is even started. Does anyone have any constructive ideas on how to fix this?

The easiest solution that comes to mind is making the play procedures clear about players not playing-out the scene here, just stating the goal, and telling the GM (er, Mastermind) to enforce this. A lot of games do this with explicit stakes-setting and they don't have trouble with the whole scene being "played out before it is even started."

Is there any reason you want the players to set stakes before playing out the scene, though? You could also just let players dive into the scene, made dice rolls, accumulate successes or points or tokens, and then at the end of that scene spend their winnings on outcomes that make sense according to the scene they just played.

Adam,
  Thanks for checking out my post.
  Good point! I don't want to set Stakes at all. All I want is for the player to say, why is this character in this scene. But when I phrased it that way, I got totally practical answers (e.g., I just walked through the door, that's why they are in the scene).
  Like a good example might be "I want to see what I can find out about the Baron."

  You know, maybe the first three steps covers all I need from a scene setup. Basically, the scene setup rules are hear to prevent player wanting to research a new spell but ending up fighting for their life to survive against a band of thugs. I am trying to facilitate the communication of needs/wants/expectations between players and GM.
  Do you think steps 1-3 can accomplish this?

Maybe 1-3 are sufficient, only because your #1 is pretty detailed and specific.

For example: I want (1) an Exploration scene, set in (2) Druid Hill Park (3) on Halloween night. Does that give you all you need?

Adam,
  Yeah, it does.
  I playtested it last night and the first three questions covered everything I wanted.
  Thanks for your help!
Dave M