And what are the exact errors you get when the plugins are disabled?
And have you created a totally default style to see if that helps?
To create a default style, create a new style with no parent:
Styles & Templates > Style Manager > Add New Style
Parent Style: No Parent Style
Title: Default vBulletin
Allow User Selection: Yes
Save
Set this as the default in AdminCP > Settings > Options > Styles & Languages > Set Default Style. Then go to Maintenance > Clear System Cache. Then browse the site using that totally default vbulletin style - do you still have the same problem?