When I facilitate strategic planning (or vision, or thinking, or whatever someone wants to call them) retreats, the first thing I announce is that we are NOT going to spend an entire day (or even an entire 10 minutes) debating their mission and vision statements. Typically the room bursts into applause. It's probably the biggest turn-off of an entire planning session because it's the ultimate in tedious word micro-management and rarely results in something better than what existed before the whole discussion started. [Note: I don't surprise the organization - so staff/president consent in advance - i.e., before I agree to facilitate.]
I've collected many mission and vision statements in the past and find that most could be accomplished with one of those little boxes of refrigerator word magnets where a thousand organizations just rearrange exactly the same words to describe very general directions in what they are and what they want to be anyway - so just about anything in the plan can fit into those regardless of what "this versus that" words are used.
These are the most common debates (regardless of industry) - should we use the word consumer or public; should we say partner or advocate; should we say innovative or state-of-the-art and add we're visionary too; how to do we word our legislative/regulatory involvement to show we're balanced and proactive; etc. etc. etc. Those were the same debates 20 years ago too. It's very likely your bylaws starts with your purpose for existence, and that's what was filed with the IRS, so if you're unclear about the general reason for existence, how about using that. Or assuming most already have mission and vision statements how about just letting them be? Did either ever really stop your organization from being able to do something at any point in time? If not, then stop the unnecessary word-play.
Instead, start out by talking about the business of your members, their challenges, where believe the world is headed for industry, and then move into what role the association should play or not play in response. That will give you the answers you need.
Your organization's mission and vision should not be so unstable that they require annual or bi-annual scrutiny or over-haul. The plan itself absolutely needs constant scrutiny, but not a mission. If you have to have the mission/vision wording debate at least add the thought that it should stand some test of time (or how valid are they really if not good enough to last a year) -- say 5 years or 10 years before "needing" to revisit? And then let the time pass. Please. Let your planning committee work on real issues.
This is what associations need to do: "Find out what needs to be done. Then do it."