Google pages indexed drop from 60K to 8K since December

What could cause this?

Hello Len ,

Number of indexed pages doesn't matter at all.You shall focus on SE referral numbers. Do you see any increase or decrease within your SE referrals ?

If by referrals, you mean how many visitors come from Google, no change that I can see. Explain to me, then, how Google finds my pages if they don't have them indexed. Does that mean we shouldn't even bother looking at the indexed stats?

You should use the indexed stats as a guide only. You should refer to the indexed page count listed in your Google Webmaster Account:
Google Webmaster Tools - Recommended for All vBSEO Customers

The site: query fluctuates and is known to be inaccurate. A site may go from 1,000 to 100,000 indexed pages over night and then back again. It's not that they lost 90,000 pages. Google is just not reporting them. This happens with all forums. Everyone will see fluctuations.

When your actual traffic changes, that is vitally more important.

Google is just not reporting them.

Do you mean Google still has them for its own use but doesn't bother telling the rest of us an accurate number of how many they are working with?

I took your suggestion and just went to look at my Webmaster TOOLS account. It tells me

You have not submitted any Sitemaps. Submit a Sitemap to help Google discover pages our crawlers might not otherwise find. Once you create and submit a Sitemap listing the URLs on your site, we'll provide you with data on how Google is indexing those pages. More information

Which is a shock to me. Doesn't the daily email report I get tell me that vBSEO has sent a map to Google?

Please add your sitemap into your webmaster tools account, and update us on the status.

You should manually submit sitemap if you want to track it in Google webmaster account.

Do I do that just one time?

Yes:
Google Webmaster Tools - Recommended for All vBSEO Customers