This is just superfish-vertical.css with a few lines added so the "supersubs-like" effect also applies to the top level menu. Comments in uppercase explain the additions.

superfish-vertical-supersubs.css

/* adding sf-vertical class in addition to sf-menu creates a vertical menu */
/* eg. <ul class="sf-menu sf-vertical"> ... */
.sf-vertical {
  min-width: 10em; /* <--- MAKE THIS MIN-WIDTH */
  *width: 10em; /* <--- ADD FALLBACK WIDTH FOR IE7 */
}
.sf-vertical ul {
  left:	100%;
  top: 0;
}
.sf-vertical li {
  width: 100%;
  float: none; /* <--- ADD THIS TO LIMIT MENU WIDTH TO WIDEST ITEM */
}

/*** alter arrow directions ***/
.sf-vertical .sf-sub-indicator {
  background-position: 0 0;
}

.sf-vertical li:hover > a .sf-sub-indicator,
.sf-vertical li.sfHover > a .sf-sub-indicator {
  background-position: -10px 0; /* arrow hovers for modern browsers*/
}