What is the first thing you want to do if you are inserted into a working project that lacks defined scope, vision or objectives? My answer may vary from yours because I don't believe the best answer is "to fill those gaps". The first course of action is to assess the why and how the project got this far without adequate scope, vision or objectives. This isn't to lay blame but to determine the best method of filling those gaps and minimize the potential negative impact to the project.
As a business analyst, you are familiar with various techniques of determining root cause. As I alluded to earlier, you have to make sure that no one interprets the root cause analysis as a way to assign blame. I cannot help but think back to a scene in an obscure movie called "Mystery Men”. The movie is about a bunch of super hero wannabes that find a scientist that creates non-lethal weapons. They accidentally activate a weapon called a blame thrower and the following is a small piece of the following dialog:
What do you call this?
Careful, careful. That's a blame-thrower.
(weapon is activated and pointed at two team members)
Your bad attitude is hurting this team!
Stop spitting! I already took a shower!
You need another one, 'cause you stink!
I have to admit that when I'm doing a root cause analysis, I imagine a blame thrower is being handled by someone in the room. If the blame thrower is activated in this setting, all of a sudden everyone has one and they may be firing them at will. For this reason, I tend to do root cause analysis in a one-on-one interviews and exclude it from my published agenda.
In this situation, your root cause technique needs to be adaptive but still be effective in a short amount of time. I tend to try something on the lines of:
1. Determine high level vision, scope and objective options
2. Discuss this in interviews but quietly challenge feedback with soft 5 whys technique
3. One of those soft whys needs to be "why wasn't this discussed before?"
4. Gather the data, analyze the data, make scope/vision/objective recommendations and a low impact plan for integration into the project
5. Iterate as needed until you get the best scope, vision and set of mutually agreed upon objectives possible.