Sorting by Title for Post Archive Categories for Custom Post Type

Question

I have a custom post type - vendors. They have a taxonomy vendor_category. I am attempting to sort the archive pages by the title (since it defaults to post date). Here is the code that I am putting into my functions.php file.

function sort_vendors_by_title( $query ) {
    // check if we're on the vendor_category archive page for the vendors post type
    if ( $query->is_main_query() && /*is_post_type_archive( 'vendors' ) &&*/ is_tax( 'vendor_category' ) ) {
        $query->set( 'orderby', 'title' );
        $query->set( 'order', 'ASC' );
    }
}
add_action( 'pre_get_posts', 'sort_vendors_by_title' );

That middle check - which I thought I would need (is_post_type_archive) always returns false. It doesn't make sense to me. I am on an archive page (vendor_categories) for a vendor type.

What am I not understanding here? (With it commented out, it does work. But, I like to be explicit in the types I'm selecting.)