Loaded FC 6.06 With Paul's 3.8 integration. Everything worked for me but not my members. (invalid user name or password)
My remedy:
During the install of tufat's FC I selected my version vb 3.8 I then took Paul's vbulletin40.php and copy and pasted everything from
// Initialise CMS // on down,, to my vbulletin38php and click save. The key element I found was to leave all the files in the inc/
cmses folder in tact. I tried just leaving the vbulletin38.php stand alone but what happend was the buttons and input area did not load.

So I put all the files back and boom everything worked.
I'm no coder so I could not tell you why it works for me and my members now but it dose. I'm using the free version for now to evaluate it. So far so good.
Thanks Paul Great work m8