[@netweb Keymaster 10 years, 8 months ago]

There are no themes or plugins that I know of that support _bbp_old_user_id, bbPress offers that and exposes it for developers to use as they will.

I pretty much cannot see any use case where displaying the old vBulletin user ID would be required, it really would only be needed to map previous vBulletin extensions and plugins to bbPress after migration.