There is no rule for even or uneven number of blades and there are opinions favoring both options as some say an even number passes the bridge simultaneously and others say an uneven number cause a rocking motion.
Vibration trips indicate something much more intense than a fan passing the bridge IMHO. I would first inspect the blade pitch of the blades my notiong if all blades pass the same point on the fan casing when the fan is spinning very slowly.
Some fans have a tracking adjustment that bring an extra variable to the equation, just to be clear, despite the fact I am mentioning this does not mean that Howden fans require tracking adjustment.
If this does help, a vibrational analysis would seem in order to troubleshoot the issue.
