The Hanoi Opera House is located near Hoan Kiem Lake on August Revolution Square. There are superb views of the opera house at the Hilton Opera House hotel which is very close by.
You will need to purchase tickets to see the inside of the opera house although there are usually a host of concerts to choose from including drama, ballet and symphonies in addition to opera. The Hanoi Opera House is also the first choice for international meetings and conferences so check online when you are visiting to ensure a performance is scheduled.