Wrong post author after import from IPB

I have deep problems with users:
the IPB 'member' table was like this:
1 - administrator
2 - user2
4 - user3
6 - user4
7 - user5

after import, VB 'user' table was so:
1 - administrator
2 - imported_administrator
3 - user2
4 - user3
5 - user4
6 - user5

all posts where had wrong author, IPB user2 to VB imported_administrator, IPB user4 to VB user5 and so on.
I manually edited IPB 'members' table to create new users and have a correct sequential number, and deleted administrator, who is the same as VB administrator.
This solved partially the problem, because I found that some posts still belong to wrong author.
In the VB 'post' table, I see the correct 'username' for those posts, but the 'userid' is wrong.

thank
Roby