How am I going to solve this (message board disaster related)

[FleaBag]

I would do it how you suggested - import into a seperate vB, remove all other posts, users (with the exception of the users who posted in these threads) and groups e.t.c. and then use Impex to import.

On a site point, you are under no obligation to remove the posts - personally I don't remove any posts unless they are of an illegal nature.