Drop in Google SERPs; wondering why

After 2+ years of slow and steady improvement in SERPs, we took a fall a couple of weeks ago. Terms where we had #1 and #2, now return only #8 or #9. Terms where we were #5 or #6, have dropped to #30-#40. And some terms, including a handful we really care about, fell from page 1 to off the map completely. Our page views are down around 20%.

Clear we were hit with a penalty of some type.

Where would you start looking to try to uncover the penalty and reverse the downward trend? I do my own SEO, but am considering hiring a consultant. I am concerned that sales will take a hit if this keeps up.

I don't think we have any bad outbound links and all of our SEO is very white hat. Page optimization, good links, vBulletin forum, quality html pages, no keyword SPAM, etc.

Where should I start?

FB

How long have you had vbseo installed?

The fall came before vBSEO. I only installed it a few days ago. Of course I must be doing something else wrong with the site to get hit with a penalty.

Duplicate content?
FB

Could it be that duplicate content urls are being consolidated? Something like www.mysite.com and mysite.com being reduced to only www.mysite.com in the serps...but sitewide.

Good idea. I did the rewrite that forwards everything to www.myself, and even used the Google webmaster tool to set that option as well. If that was the problem, I think I took care of it.
FB