I've not really done this before, but perhaps you could try the following...
var userName=context.getUser().getCommonName();
var lastName = (userName.indexOf(' ')==-1) ? userName : userName.split(' ')[1];
var initial=lastName.charAt(0);
var v:java.util.Vector = new java.util.Vector();
v.addElement(initial);
v.addElement(lastName);
return v;
I haven't tested this, but it essentially breaks down the user's name into teh components you require and adds them into the vector.
I hope this helps!