(I'm going to speak out of my experience with importing from phpBB3)
Few reasons why I'm write this:
I always forget to do those things... & how to do it...
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The story:

I just did a new test part, and saw that after the import, only stickies were showing... I redid a few modules time after time, and still the same... Even reinstalled vB... but still the same... Then re-researched the impex documentation, couldn't found anything... then searched this forum... And finally saw something...
https://www.vbulletin.com/forum/show...=1#post2229387
but thought by myself... Am I going to need to do that for each forum??? there must be an easier way to do that....
That remembered me to the following:
https://www.vbulletin.com/forum/show...=1#post1507930
So to make it that each forum shows all threads, you can execute a queries that makes that happen...
Automatic Query: "Show All Threads"
Originally posted by Jake Bunce View Post
Run these queries:
Code:
UPDATE forum
SET defaultsortorder = 'desc'
Code:
UPDATE forum
SET defaultsortfield = 'lastpost'
But still, it wouldn't worked... and redid those queries a few times... And thought, how the hell doesn't it work???
Re-read that post and saw:
Originally posted by Jake Bunce View Post
Then you will need to update your forum info:
Admin CP -> Maintenance -> Update Counters
So that's it for the story... (later about attachments...)
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Step-by-step part (how I (will) do it):
After importing

Admin CP -> Maintenance -> Execute SQL Query -> Automatic Query: "Show All Threads" (this will make that each forum shows all threads (instead of only threads from last month))
Admin CP -> Maintenance -> Execute SQL Query -> Manual Query:
Code:
  UPDATE forum
  SET defaultsortfield = 'lastpost'
Admin CP -> Maintenance -> Execute SQL Query -> Manual Query:
Code:
 UPDATE forum
 SET defaultsortorder = 'desc'
And NOT to forget!!!:
Admin CP -> Maintenance -> Update Counters
("Rebuild Thread Information" & "Rebuild Forum Information") (thats what I personally do...)