Same problem here. I think it's related to special characters.
Possibly adding something like
Code:
$pmText['touserarray'] = $this->_convertToUtf8($pmText['touserarray']);
might help. Just guessing here though as I cannot test this right now as I just went ahead with importing threads and posts and this step is not finished yet.
P.S. you might want to check XenForo AdminCP -> Tools -> Server Error Log for details. What I posted above is related to the error on line 1058 while importing PMs: library/XenForo/Importer/vBulletin.php(1058)