There are two main things you'll probably notice about the architecture in Hanoi.  Firstly there's the foreign influences, particularly from the French colonial period, and secondly, that many houses and buildings in Vietnam seem seem to be very tall and narrow.  The reason for this is the way people are/were taxed on property - by the width of the front of the building. This is especially apparent in the Old Quarter of Hanoi (36 Pho Phuong) where people buy houses with very narrow frontages so that they minimise their tax burden while having a place to display their merchandise to passers by. These buidlings are reffered to as "tube houses" and often include courtyards partway through to improve air flow. Other interesting architectural  landmarks include government buildings such as the History Museum, Hanoi Opera House, National Bank, the Foreign Ministry, and the Ho Chi Minh mausoleum, Saint Josheph Cathral...

Furture architecture and expanding: Hanoi has just expanded it area to the west (include the old Ha Tay province, part of Vinh Phuc) and will encourage resident to move there. Currently french style old apartment will be replaced by new modern Korean and Japanese style building. A very large project of building the red-river city is also in research.