To restore original style/template you have to create new style without parent:
Go to you vBulletin admin section.
Click on 'Style Manager' under 'Styles & Templates' in the left nav menu.
Click 'Add New Style'
No Parent Style
Style title ( for example: Default vBulletin )
Choose 'Yes' or 'No' to allow users to (not)select this style when visiting your forums.
Style display order - in what order the style shows in the Style Manager in the Admin CP.
Now you have original default style as it was right at the beginning.
If you don’t want to keep the other modified style you had you can delete it under the Style Options or you can keep it if you may want to use it in the future. Keep in mind that this new style is totally default so you need to add your logo and other things again.