Here's an example to get you started. If you change the width of the submenu element, you can adjust it to be wide enough to fit the number of elements you want and still look good in the theme. Note that this only accounts for 2 levels in the menu.
#access ul ul {
width: 376px;
}
#access ul ul a {
width: auto;
}
Changing the layout of a menu can be a slippery slope. Sometimes you can just change it a little bit and it will work for your specific custom menu, however, if you have a complex menu with a lot of items and levels, it could actually end up being quite tricky and a lot of work to change from one menu layout to another. I would recommend experimenting a little with just the small example I posted above to see if it will work and make sure to test with your final menu layout.
If you need to refer to how the menus are setup for both Manifest and Vertigo, you can check the original stylesheet for each theme and find the menu or main navigation section.
Here is the stylesheet for Manifest:
https://s1.wp.com/wp-content/themes/pub/manifest/style.css?m=1339795534g&minify=false
Here is the stylesheet for Vertigo:
https://s1.wp.com/wp-content/themes/pub/vertigo/style.css?m=1339795534g&minify=false