You can implement this as an extension of the "home" icon. Basically, you need to enter two classes into the menu item "class" option - home calendar
Once you've done that, you'll need this CSS code. You can either put it in the Custom CSS area in Vamtam/General or in a child theme http://vamtam.com/child-themes :
#main-menu .menu .menu-item.home.calendar > a:before {
content: "ICON CHAR";
font-family: icon-font;
}
The theme has demo page with a full list of icons. Find the one that you want, open the developer tools of you browser of choice, and copy its symbol.
As for the font-family, the rule is:
- if an icon's name starts with theme- - then use font-family: theme;
- all other icons the font-family: icomoon;