Google listings

We are planning to convert to vBulletin at the end of this month.
Our Google listings stand at 21,000+ for the current phpBB installation.

Can anyone tell me what will happen to the listings when we go live with vBulletin.
Any advice appreciated

They'll all be dead links most likely. I don't think Impex keeps the thread id's the same when it imports from phpBB, so I don't think you'll be able to make a rewrite rule to make the old thread links redirect to the new ones. Check though, because if the id's are the same, you should be able to add some lines to .htaccess to 301 redirect the old URLs to the new ones.

And make sure you get the vBSEO Sitemap up and running and submitting as soon as you convert so your new URLs will start getting spidered by Google.

The thread id's are indeed different.

I plan to close the forum by removing the log in and all the links to various other related functions. Just leave the forum in a read only state.
I think six months or so and then perhaps delete the forum.

Should I place a redirect to the new forum and where ?

As there will be no new content on the old forum, would it be advisable to block spiders.
If so what is the best way to do this ?