I hope I can jump on this because I was having the same problem.
Originally my user database was tied to phpbb and I changed it to vBulletin.
It took some editing using phpMyAdmin to get everything working.
it seemed to be a Catch-22 trying to change the setting in the admin section.
Anyway. . .
I edited the cookies settings as above.
My problem is that I can't log out of PP unless I log out of vB