vBulletin 3 was originally coded 16 years ago when there weren't even such things as % or rem or em units used in styling. It was coded using transitional HTML 4.0 and CSS 2.0, both of which are years deprecated. Most of the "responsive" styles you will find are not responsive styles at all but rather hack attempts at making a fixed width style flex.
Mark's assessment is correct. You will need to write a style from the !DOCTYPE to the closing HTML tag if you want it to function like modern responsive themes. In order to make any existing theme function properly you would have to rewrite it to use HTML 5 and CSS 3 anyway.
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