Duplicating single.php to get unique display based on categ

I’m using Responsive Theme. I’ve posted this question in responsive theme’s forum and My response was that they do not support WP system files.

I have special posts categorized as vendors by using categories of posts. I want a customized display of these vendors unlike the default while keeping the default display available for standard posts. I’m currently using “Content/Sidebar Half Page” in the layout settings of the vendor posts and a standard post is using the “Full Width Page (no sidebar)”. I can ensure that my vendors will be the only posts that use “Content/Sidebar Half Page”.

I’ve determined that all posts are using “single.php” to render, but I can’t find any CSS identifiers in the page that result from using “Content/Sidebar Half Page“ that I can hook on to start styling the vendors display the way I want.

How can I can give my categorized posts a custom display? I’m open to new ways of doing this so all suggestions are welcome