Generally speaking, if you narrow a browser window and it adds a horizontal scrollbar, or if there is an apparently empty area on the right-hand side on a tablet or phone, then it means that there is some content on the page that is fixed width, i.e. content that is not responsive and so will not reduce its width as the browser window is made narrower.
This basic issue crops up again and again in our Help Center, and in 100% of cases the problem has turned out to be with the content, nothing at all to do with the Vellum theme or any plugin, just purely caused by the website designer adding content that is not responsive and is too wide to fit in the smartphone screen or into a narrow column.
If you find yourself in this situation you will need to go through your page content looking for any element that doesn’t scale or reduce in width responsively.
If you can post a link to a live web page that displays the issue I’ll gladly take a look.