The key was to find the css file with the class that contains the search_quickhits positioning. For the Vector template this is in ../lib/tpl/vector/static/3rd/dokuwiki/design.css, so I changed the width from 30% to 100% to do the trick. I realize this might get overwritten when the template is updated, so at a worst case I may have to re-edit the file. Works for me so far.
div.dokuwiki ul.search_quickhits li {
margin: 0 1.0em 0 1.0em;
float: left;
width: 100%;
}