Most of the train companies across Europe release their tickets around three to six months in advance, many of which can be cheaper the earlier you book. If you know the dates you want to travel, you may be able to find some cheaper train tickets from Milan Malpensa Airport to Vergiate by booking early.§
Many of the train services in Europe are also popular commuter services, lots of train companies increase ticket prices during “peak hours” (generally between 06:00 – 10:00 and 15:00 – 19:00 on weekdays). If you can, consider travelling outside of peak hours to find lower priced tickets.
On some of the busier routes, you might also have the option to take a slower or connecting train. It may take a little longer than some high-speed or direct services, but if you have a little extra time on your hands, you might find a cheaper fare. Plus, you'll have more time to enjoy the view of the countryside!