The get() method of Vector returns an Object, so to get your Item back you must cast it first:

Code:

Vector v; 
for(int i=0; i<200; i++){ 
  v.add(new Item()); 
}  

for(int i=0; i<v.size(); i++){ 
  Item item = (Item)v.get(i);
  item.methodName(); 
}
 

Unfortunately, to do it on one line now looks like this:

Code:

  ((Item)v.get(i)).methodName(); 
 

It's one of the long standing uncomfortable aspects of Java, but it's one that becomes familiar with time.  There are various syntactical ways this is solved in the latest versions of the Java language, but they aren't in Processing yet.

It's also worth noting that if you want Vector to work across all versions of Java, you should use elementAt instead of get, and addElement/removeElement instead of add/remove.  I'm not sure what version of Java the latest version of Processing targets though, so that might be irrelevant.