Not a good idea, as then anyone could login as anyone in the old forums and see their private messages.Just catching up on this thread. I think what we may want to to do in the short run is simply reset all the user passwords and emails to something super generic, and make everyone's permission read only. That way the data integrity is maintained, but the security risk is mitigated.
What I would do: configure a HTTP redirect from https://www.vcfed.org/forum to https://forum.vcfed.org, and nothing else. Old links will die, but (1) all content has been migrated to the new forums, and (2) given a little time, all content will be reindexed by Google in its new home.